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Douglas Harper's Etymology Dictionary

co-

in Latin, the form of com- in compounds with stems beginning in vowels and h- and gn- (see com-). Taken in English from 17c. as a living prefix meaning "together, mutually, in common," and used promiscuously with native words and Latin-derived words not beginning with vowels, sometimes even with words already having it (such as co-conspiritor).

Wiktionary

co-

pre. 1 together; mutually; jointly 2 partner or subordinate in an activity 3 to the same degree 4 (context mathematics English) of the opposite, of the counterpart

The Collaborative International Dictionary

Co-

Co- \Co-\ (k[-o]-). A form of the prefix com-, signifying with, together, in conjunction, joint. It is used before vowels and some consonants. See Com-.

Longman Dictionary of Contemporary English

co-

prefix
EXAMPLES FROM OTHER ENTRIES
▪ a coeducational school
▪ the copilot