CO-

prefix 1 added to: a nouns, with the sense 'joint, mutual, common' (co-author; coequality). b adjectives and adverbs, with the sense 'jointly, mutually' (co-belligerent; coequal; coequally). c verbs, with the sense 'together with another or others' (cooperate; co-author). 2 Math. a of the complement of an angle (cosine). b the complement of (co-latitude; coset). [orig. a form of COM-]