Glossary
combining vowel
A combining vowel — almost always o — is the vowel placed between a root and a suffix when the suffix starts with a consonant.
A combining vowel is the vowel, almost always o, that sits between a root and a suffix or between two roots to keep the word pronounceable. The rule is mechanical: a suffix that begins with a consonant takes the vowel, so gastr/o/scopy rather than gastrscopy; a suffix that begins with a vowel does not, which is why it is gastritis and not gastroitis. The vowel is there for sound and carries no meaning of its own, which is the point to hold on to — an o in the middle of a medical word is usually not telling you anything about the body. When a question asks you to build a word from parts, the vowel's presence or absence is not usually the thing being tested; the root and suffix are. Do not confuse the combining vowel with a root vowel or with the o that belongs to a prefix.
Taught in
Related terms
- root — The root is the part of a medical word that says what the word is about — the organ, the tissue or the substance — and everything else in the word modifies it.
- suffix — A suffix is the part fixed to the end of a medical word, and it says what is being done or what kind of word this is — a condition, an inflammation, a procedure, a specialist.
- prefix — A prefix is the part fixed to the front of a medical word, and it changes direction, size, speed, number or negation without changing what the word is about.
- word part — A word part is a prefix, root, suffix or combining vowel — one of the pieces a medical word is built from.
Source: MedlinePlus (US National Library of Medicine) consumer guidance on understanding medical words, which the vocabulary guide follows for the prefix, root, suffix and combining-vowel system. US federal government material, not subject to copyright in the United States. Every definition and example above was written for this course in our own words; no dictionary or glossary text is reproduced. (MedlinePlus accessed 2026-10-27; definitions written 2026-10-27)