co-

co-

pref.

1. Together; joint; jointly; mutually: coeducation.

2.

a. Partner or associate in an activity: coauthor; cofounder.

b. Subordinate or assistant: copilot.

3. To the same extent or degree: coextensive.

4. Complement of an angle: cotangent.

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[Middle English, from Latin, variant of com-, com-.]

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co-

prefix

1. together; joint or jointly: coproduction

2. indicating partnership or equality: co-star, copilot

3. to the same or a similar degree: coextend

4. (in mathematics and astronomy) of the complement of an angle: cosecant [Latin; see com-]