CO-

var. of com- before a vowel, h, and gn: coadjutor; cohabit; cognate. The prefix co- now productively forms new words from bases beginning with any sound ( co-conspirator; co-manage; coseismic ), sometimes with the derived sense "auxiliary, subsidiary" ( coenzyme; copilot ), and, in mathematics and astronomy, with the sense "complement" ( codeclination ).