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Singular and Plural Forms

Author: Sophia
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what's covered
In this lesson, you will learn about singular and plural forms of common medical terms. You will learn rules that will help you produce a wide variety of plural forms of common medical words. As you work through examples, you will see how to use the rules correctly and how errors can occur. Specifically, this lesson will cover:

Table of Contents

1. Singular and Plural Forms

When using medical terminology, you will regularly need to choose between singular and plural forms. Use the singular form for single objects (e.g., one bacterium) and the plural for multiple objects (e.g., several bacteria).

Correctly choosing the singular or plural form is essential for clear communication and accurate documentation. It helps in correctly interpreting test results (for example, knowing if a patient has one diagnosis or multiple diagnoses). Correct word choice reduces errors in prescriptions and medical orders.

Sometimes, the plural forms may not be what you expect. Remember that many medical terms originate from Greek and Latin words, which have different plural forms.

A microscope is shown on the left side of the screen, with a line leading from the stage of the microphone that expands into a large circle in the center of the screen, indicating what is inside the circle is being viewed through the microscope. The large circle contains a bacterium that’s slightly waving as if in fluid, represented by a blue oval and three tail-like appendages off the bottom of it. The word ‘Bacterium’ is shown to the right of the large circle. The word ‘Bacterium’ splits apart to become two parts, ‘Bacter’ and ‘-ium’, where the suffix ‘-ium’ turns purple. The suffix ‘-ia’ floats in, in purple, to replace the suffix ‘-ium’. Six more bacteria appear inside the circle to join the original single bacterium. The prefix and suffix join to form the word ‘Bacteria’, and text appears below this word ‘Plural of bacterium’. The word ‘Bacteria’ then splits apart to become two parts, ‘Bacter’ and ‘-ia’, where the suffix ‘-ia’ turns purple. Six of the seven bacteria inside the circle fade out and disappear, leaving a single bacterium in the center. The word ‘Bacteria’ splits apart to become two parts, ‘Bacter’ and “-ia”, where the suffix “-ia” turns purple and fades out, and the suffix “-ium” floats in, in purple. The word parts ‘Bacter’ and ‘-ium’ rejoin to make the single word ‘Bacterium’ in black as the single bacterium remains on the screen in the circle.

Each table below shows examples of medical terms that follow a specific rule for forming plurals. There are also opportunities for you to try it for yourself.

Singular Words Ending in “-a” Change to “-ae” in Plural
Singular Plural Meaning
Vertebra Vertebrae Bone(s) of the vertebral column (spine)
Patella Patellae Kneecap(s)

try it
Let’s try an example.
What is the correct plural of bursa (a sac found in joints)?
The correct word is bursae. For example, “The bursae in the patient’s shoulders are inflamed.” This condition is called bursitis.

Singular Words Ending in “-us” Change to “-i” in Plural
Singular Plural Meaning
Bronchus Bronchi Major branches of the human respiratory tract; large airways of the lungs
Fungus Fungi Members of the group of organisms that includes molds, yeasts, and mushrooms
Nucleus Nuclei Central part(s) of many cells, including human cells (but not bacterial cells); contains chromosomes consisting of DNA and proteins

hint
Remember that not all words follow each rule, so always check when learning new terms.

For example, the plural of virus is viruses, not viri. Viruses are nonliving agents that can infect cells; some cause disease (e.g., influenza and the common cold).

try it
Let’s try an example.
What is the correct plural of alveolus (a sac found in the lungs where gas exchange takes place)?
The correct word is alveoli. For example, “Oxygen moves from air into the blood in capillaries around alveoli in the lungs.”

Singular Words Ending in “-um” Change to “-a” in Plural
Singular Plural Meaning
Bacterium Bacteria Single bacterium (a single-celled organism) to multiple bacteria
Ovum Ova Female reproductive cell(s)
Datum Data One piece of information; many pieces of information

did you know
Note that the word data is an unusual case. In everyday writing, “data” is regularly written as a singular term. In medical writing, data may be used as a plural term. Be careful to look at the context to make sure that you understand how the term is being used. Below are a couple of examples. Depending on the context, either usage may be acceptable.

“The data shows that patients have been waiting for less time” versus “The data show that the patients have been waiting for less time.”

“They were only able to obtain a single datum from the recorder; it is not functioning correctly.”

try it
Let’s try an example.
What is the correct plural of diverticulum (a pouch-like intestinal structure)?
The correct word is diverticula. For example, “Inflammation of diverticula is called diverticulitis.”

In addition, some Latin-based terms end in -ex or -ix and change to ices.

Singular Words Ending in “-ex” or “-ix” Change to “-ices” in Plural
Singular Plural Meaning
Cervix Cervices Lower part of the uterus or multiple of this body part; connects the uterus to the vagina
Appendix Appendices Small, pouch-like structure(s) attached to a larger pouch that opens to the large intestine near the junction with the small intestine
Index Indices Record(s) or list(s)

try it
Let’s try an example.
What is the correct plural of varix (a type of abnormal vessel)?
The correct word is varices. For example, “Varices can form in the esophagus.”

Some medical terms ending in -is change to -es to form the plural. This is notable in frequently used terms like diagnosis and prognosis. Diagnosis is a term with complex origins, including both Latin and Greek (Oxford English Dictionary, 2025).

Singular Words Ending in “-is” Change to “-es” in Plural
Singular Plural Meaning
Diagnosis Diagnoses Term(s) identifying a disease (plural diagnoses)
Prognosis Prognoses Prediction(s) of possible disease outcomes

Some medical terms change from -on to -as in the plural form.

Singular Words Ending in “-on” Change to “-a” in Plural
Singular Plural Meaning
Ganglion Ganglia Collection(s) of nerve cell bodies in a specific part of the nervous system
Criterion Criteria Standard(s) for evaluation

try it
Let’s try an example.
What is the correct plural of atrium (one of the two upper chambers of the heart)?
The correct word is atria. For example, “The atria appeared normal based on echocardiography.”

The next set of words change from -ma to -mata.

Singular Words Ending in “-ma” Change to “-mata” in Plural
Singular Plural Meaning
Sarcoma Sarcomata A type of tumor (or tumors) that develop in connective tissue
Carcinoma Carcinomata A type of malignant tumor (or tumors) that forms in epithelial tissue (tissue that forms linings, such as the outer layer of skin and the inner lining of the digestive tract)

did you know
In the United States, the plural for hematoma is generally hematomas. However, the term hematomata is also acceptable (Merriam-Webster Dictionary, n.d.).

There are still more rules to learn. For example, the common term biopsy, which means to take a sample for testing, or a sample taken for testing, has the plural form biopsies.

Singular Words Ending in “-y” Change to “-ies” in Plural
Singular Plural Meaning
Artery Arteries Relatively large blood vessel(s) that carries blood away from the heart
Biopsy Biopsies Sample(s) taken for testing, such as analysis to detect cancer cells

try it
Let’s try an example.
What is the correct plural of therapy?
The correct word is therapies. For example, “The patient needed multiple therapies during recovery.

Some words end in x, and these may be more unfamiliar in their plural forms. Note that an -x ending in an English based word suggests that the plural will be -ces, as shown in the table below.

Singular Words Ending in “-x” Change to “-ces” in Plural
Singular Plural Meaning
Thorax Thoraces Chest region(s)
Pharynx Pharynges Region at the back of the throat connecting the rear of the nasal cavity (nasopharynx), rear of the oral cavity (oropharynx), and esophagus; multiple of these regions
Larynx Larynges Voice box(es)

Finally, some words end in –eau and change to –x in the plural, as shown in the table below.

Singular Words Ending in “-eau” Change to “-x” in Plural
Singular Plural Meaning
Bureau Bureaux Administrative office(s) or department(s)
Plateau Plateaux In medicine, reaching a stable level (singular) or stable levels (plural)

key concept
Some English words have irregular plural forms.

Tooth → Teeth
Foot → Feet
Child → Children

A single anatomical vertebra is shown on the left side of the screen, while the word ‘Vertebra’ is shown in large black font in a gray box on the right side of the screen. The word ‘Vertebra’ splits apart where ‘Vertebr’ remains black and the suffix ‘-a’ turns purple. The suffix ‘-a’ transforms into the suffix ‘-ae’ while the image of the single anatomical vertebra transforms into an entire spinal column, also depicted anatomically, and shown within the silhouette of a human torso. The suffix ‘-ae’ joins to the prefix ‘Vertebr’ to form the word ‘Vertebrae’ with the whole word turning black. Underneath the word ‘Vertebrae’ appears the words ‘Plural of vertebra’, in smaller text. The word ‘Vertebrae’ splits apart again where the suffix ‘-ae’ splits off and turns purple, and the view of the entire spinal column zooms back in to a single vertebra. As the zoom in to a single vertebra occurs, the suffix ‘-ae’ transforms to ‘-a’, remaining purple. The suffix rejoins to make the full word ‘Vertebra’ in black.

2. Common Mistakes

As you get used to converting singular forms to plurals and vice versa, be careful to avoid common pitfalls. Here are some examples of common mistakes.

Incorrectly applying standard English pluralization rules to Latin/Greek words:

EXAMPLE


Incorrect: Diagnosis → Diagnosis's
Correct: Diagnosis → Diagnoses

Confusing "-us" and "-um" endings:

EXAMPLE


Incorrect: Bacterium → Bacteriums
Correct: Bacterium → Bacteria

Mistaking English plurals for medical ones:

EXAMPLE


Incorrect: Thorax → Thoraxes
Correct: Thorax → Thoraces

Using incorrect pluralization for exceptions:

EXAMPLE


Incorrect: Virus → Viri
Correct: Virus → Viruses

IN CONTEXT

Let’s compare some correct and incorrect terms in medical records and documentation.

Incorrect: A physician notes, “The patient has three vertebra misaligned.”
Correct: A physician notes, "The patient has a single vertebra misaligned." (singular).
If multiple bones are affected: "The patient has three vertebrae misaligned." (plural).

Incorrect: A single bacterium is visible: A bacteria was identified.
Correct: A test for a single bacterium: "A gram stain identified one bacterium in the sample."
If multiple bacteria are found: "Gram stain identified multiple bacteria in the sample."

Incorrect: A doctor recommends medication based on a single rule: Follow the criteria listed to decide whether to take a dose.”
Correct: A doctor recommends medication based on a single rule: "Follow the criterion listed to decide whether to take a dose.” If multiple criteria exist: "Follow the criteria listed for dosage.”

summary
In this lesson, you learned about singular and plural forms of common medical terms. In particular, you learned rules for terms with varied origins, including Latin and Greek origins. After reviewing these rules and examples of singular and plural forms, you learned about some common mistakes and reviewed examples. As you practice converting singular forms to plural and vice versa, it will become more natural to remember the forms without having to carefully review the rules each time.

Source: THIS TUTORIAL HAS BEEN ADAPTED FROM “OPEN RN | MEDICAL TERMINOLOGY – 2e” BY ERNSTMEYER & CHRISTMAN AT OPEN RESOURCES FOR NURSING (Open RN). ACCESS FOR FREE AT https://wtcs.pressbooks.pub/medterm/ LICENSING: CREATIVE COMMONS ATTRIBUTION 4.0 INTERNATIONAL.

REFERENCES

Merriam-Webster. (n.d.). Hematoma. In Merriam-Webster.com dictionary. Retrieved April 17, 2025, from www.merriam-webster.com/dictionary/hematoma

Oxford English Dictionary. (n.d.). Diagnosis. In Oxford English Dictionary online. Retrieved April 22, 2025, from diagnosis, n. meanings, etymology and more | Oxford English Dictionary

Terms to Know
Appendix

Small, pouch-like structure(s) attached to a larger pouch that opens to the large intestine near the junction with the small intestine.

Atrium

One of the two upper chambers of the heart.

Biopsy

Sample(s) taken for testing, such as analysis to detect cancer cells.

Cervix

The lower part of the uterus or multiple of this body part; it connects the uterus to the vagina.

Criterion

Standard(s) for evaluation.

Datum

One piece of information; many pieces of information.

Fungus

Members of the group of organisms that includes molds, yeasts, and mushrooms.

Index

Record(s) or list(s).

Nucleus

Central part(s) of many cells, including human cells (but not bacterial cells); contains chromosomes consisting of DNA and proteins.

Ovum

Female reproductive cells. 

Patella

Kneecap.

Pharynx

Region at the back of the throat connecting the rear of the nasal cavity (nasopharynx), rear of the oral cavity (oropharynx), and esophagus; multiple of these regions.

Plateau

In medicine, reaching a stable level (singular) or stable levels (plural).

Prognosis

Prediction(s) of possible disease outcomes.

Sarcoma

A type of tumor (or tumors) that develop in connective tissue.

Thorax

Chest region(s).

Virus

An acellular agent; some can cause disease.

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