before vowels coen-, word-forming element meaning "common," from Latinized form of Greek koinos "common, public, shared, general, ordinary," from PIE *kom "beside, near, by, with" (see com-).

assimilated form of com- "with, together" before stems beginning in -l-. In early Latin, com- was assimilated to these as con-, but col- later also was used. Latin words in coll- became col- in Old French and thus in early Middle English but were altered back to coll- with the revival of learning.

word-forming element usually meaning "with, together," from Latin com, archaic form of classical Latin cum "together, together with, in combination," from PIE *kom- "beside, near, by, with" (compare Old English ge-, German ge-). The prefix in Latin sometimes was used as an intensive.

Before vowels and aspirates, it is reduced to co-; before -g-, it is assimilated to cog- or con-; before -l-, assimilated to col-; before -r-, assimilated to cor-; before -c-, -d-, -j-, -n-, -q-, -s-, -t-, and -v-, it is assimilated to con-, which was so frequent that it often was used as the normal form.

word-forming element meaning "together, with," sometimes merely intensive; it is the form of com- used in Latin before consonants except -b-, -p-, -l-, -m-, or -r-. In native English formations (such as costar), co- tends to be used where Latin would use con-.

word-forming element meaning "against, in opposition," from Latin adverb and preposition contra "against" (see contra (prep., adv.)). The Latin word was used as a prefix in Late Latin. In French, it became contre- and passed into English as counter-. The Old English equivalent was wiðer (surviving in withers and widdershins), from wið "with, against."

word-forming element indicating "dung, filth, excrement," before vowels copr-, from Latinized form of Greek kopros "dung," from PIE root *kekw- "excrement." Hence, coprology "study of obscene literature" (1856).

before a vowel cosm-, word-forming element from Latinized form of Greek kosmos (see cosmos). In older use, "the world, the universe;" since 1950s, especially of outer space. Also cosmico-.

word-forming element used in English from c. 1300 and meaning "against, in opposition; in return; corresponding," from Anglo-French countre-, French contre-, from Latin contra "opposite, contrary to, against, in return," also used as a prefix (see contra (prep., adv.)). A doublet of contra-. In some cases it probably represents a purely English use of counter (adv.).

word-forming element meaning "of the skull," from Latinized combining form of Greek kranion "skull" (see cranium).

word-forming element typically representing cross as a noun, adverb (cross-examine), adjective (crossbar), and in many words a confluence of them. "There is no distinct line of division between cross as an adjective and cross as a prefix. As a prefix, it often represents the adv. cross, or the prep. cross, across." [Century Dictionary]