micro-

WordReference Random House Learner's Dictionary of American English © 2026

micro-, prefix. 

1. • micro- comes from Greek, where it has the meaning: small or very small in comparison with others of its kind:
     micro- + organism → microorganism (= very small living creature).
   • micro- also means restricted in scope:
     micro- + habitat → microhabitat;
     micro- + economics → microeconomics.
   • micro- also means containing or dealing with texts that require enlargement to be read:
     micro- + film → microfilm.
   • micro- is used to mean one millionth:
     micro- + gram → microgram.

Also,[esp. before a vowel,] micr-. 

Collins Concise English Dictionary © HarperCollins Publishers::

micro-, micr- combining_form

1. small or minute: microspore
2. involving the use of a microscope: micrography
3. indicating a method or instrument for dealing with small quantities: micrometer
4. (in pathology) indicating abnormal smallness or underdevelopment: microcephaly, microcyte
   Compare macro-2
5. denoting 10^–6: microsecond
   Symbol: μ

Etymology: from Greek mikros small

WordReference Random House Unabridged Dictionary of American English © 2026

mi•cro  (mī′krō), adj., n., pl. -cros. 
adj. 

1. extremely small.
2. minute in scope or capability.
3. Computing of or pertaining to microcomputers.
4. of or pertaining to microeconomics.

n. 

5. anything extremely small in scope or capability.
6. Computing a microcomputer.
7. microeconomics.

• independent use of micro-, or shortening of words with this initial element

micro-, 

1. a combining form with the meanings "small'' (microcosm;
   microgamete), "very small in comparison with others of its kind'' (microcassette;
   microlith), "too small to be seen by the unaided eye'' (microfossil;
   microorganism), "dealing with extremely minute organisms, organic structures, or quantities of a substance'' (microdissection;
   microscope), "localized, restricted in scope or area'' (microburst;
   microhabitat), "(of a discipline) focusing on a restricted area'' (microeconomics), "containing or dealing with texts that require enlargement to be read'' (microfilm;
   microreader), "one millionth'' (microgram).

Also,[esp. before a vowel,] micr-. 

• Greek: combining form representing mīkrós small