co-

aff.

var. of com- before a vowel, h, and gn : coalesce; cohere; cognate. The prefix co- with the sense “joint, jointly,” now forms new words from bases beginning with any sound (cochair; cogeneration; costar; coworker), sometimes with the derived sense “auxiliary” (coenzyme; copilot), and, in mathematics and astronomy, with the sense “complement” (codeclination).