I am learning medical terminology. My medical terminology textbook has me all confused about roots, prefixes, suffixes, and combining forms; so I have been doing some research. I've found that most dictionaries I've referenced have a different concept of a combining form versus a prefix or suffix. But even using the concept proposed by the dictionaries, I'm having trouble. Follow this link to see how Merriam-Webster distinguishes combining forms and affixes: https://www.merriam-webster.com/dictionary/combining%20form. For example, the online Merriam-Webster dictionary refers to 'hyper-' as a prefix, but "tachy-" as a combining form. I don't understand how Merriam-Webster came to that conclusion. Any insight you can provide would be greatly appreciated. Thanks.


There isn't really any clear definition of "prefix". However, in general, elements of complex words that are derived from nouns or adjectives are not called prefixes. E.g. the black in the English word blackbird is not considered to be a prefix.

Tachy- is from the the Greek adjective ταχύς.

Hyper- is from the Greek preposition/adverb ὑπέρ. It's fairly common for Greek or Latin prefixes to come from prepositions or adverbs.

  • I think this is a great question, but we have two opposing answers, from two well-respected contributors, each offering a reasonable argument. I’m inclined to think yours is the more correct, but it’s lacking in references, which is one of the elements we users look for in assessing how trustworthy an answer is. Can you provide any authoritative source to support your assertion about nouns/adjectives generally not forming prefixes? A link to takhys being an adjective would also be helpful. :-) – Reinstate Monica Jul 1 at 1:25
  • I assume this is the correct answer, but I think you can improve on it. The term combining form is very common when applied to the form a Greek or Latin adjective (or noun) takes when used to form compounds. Most frequently the combining form is the stem ending in the thematic vowel -o-. The combining form is thus different from the lookup form (e.g., palatum ‘palate’ vs palato-). A prefix, conversely, often has allophonic variants, but is generally identical to the lookup form of the preposition or adverb it is derived from (e.g., ad to vs ad-). – Janus Bahs Jacquet Jul 1 at 5:14
  • As you've stated, there does not appear to be a concise, or universally accepted, definition of prefix. I do agree with your assessment that origin plays a role in the categorization of a word part. As another example, malfunction and dysfunction (at least in a medical sense) seem to have very similar meanings. Yet the Marriam-Webster dictionary categorizes “mal-“ as a combining form and “dys-“ as a prefix. Perhaps, while learning medical terminology, I should worry less about categorizing the word part, and worry more about recognizing word parts and their meaning. – slax Jul 1 at 20:59

"Tachy" (from ancient Greek) means "fast" as in tachycardia, tachypnea, tachygraphy, meaning fast heart rate, fast breathing and fast writing, respectively.

prefixes are defined as "morphemes (specific groups of letters with particular semantic meaning) that are added onto the beginning of roots and base words to change their meaning. Prefixes are one of the two predominant kinds of affixes—the other kind is suffixes, which come at the end of a root word."

In medicine, both tahchy- prefix and its opposite, brady- prefix, are considered prefixes, so I was taught in Medical School.

Based on the definition of "prefix", I would say "tachy-" is a prefix and The Free Dictionary backs me up, as well as The UCL and several other sources. Whilst "tachy" is a prefix, "tachycardia" is a compound word. DC

  • MW and Collins both define it as a “combining form”, so clearly there are differing approaches. If -on is a suffix in tachyon, doesn’t that make tachy- either a root or combining form? – Reinstate Monica Jul 1 at 1:02
  • @Chappo I've edited and tried to make it clearer. – Centaurus Jul 1 at 3:00
  • Does how the word part is used determine its classification, i.e. much the same way context helps determine the meaning of a word? For example, tachy- in tachyon would be a combining form, yet tachy- in tachycardia would be a prefix. Take melan- as another example. In the case of melanin, is it a combining form, and in the case of melanoma, a prefix? – slax Jul 1 at 21:38
  • @slax There aren't many words beginning with "tachy" in English, they're all qualified as something that is fast. I would, therefore, consider "tachy" a prefix in all cases. – Centaurus Jul 2 at 1:51
  • @Centaurus True. There are few words that begin with tachy-. In fact, I only found one tachyon. And I don’t know who created the word: linguist, scientist or screen writer. It does seem that word formation can be arbitrary; i.e. not all words are created by linguist. But once a word is accepted, it is then subject to the prevailing morphological models. In the case of tachyon, it seems that the affix/root/combining form model would require tachy- to be a combining form, as -on is classified a suffix. I'm assuming the morphological model requires a word to contain a root. – slax Jul 3 at 19:22

The (vague) distinction that MW is making, as quoted by the OP, is that a "true" prefix can be affixed (with some restrictions) to a "normal" word (hence "pre-fix"), whereas a "combining form" such as "tachy-" is usually attached to another (non-word) combining form such as "-graphy".

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