Longman Dictionary of Contemporary English
The Collaborative International Dictionary
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-.
Douglas Harper's Etymology Dictionary
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).
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