Definition of co- in English:
1 In modern American English, the tendency increasingly is to write compound words beginning with co- without hyphenation, as in costar, cosignatory, and coproduce. British usage generally tends more often to show a preference for the older, hyphenated spelling, but even in Britain the trend seems to be in favor of less hyphenation than in the past. In both the US and the UK, for example, the spellings of coordinate and coed are encountered with or without hyphenation, but the more common choice for either word in either country is without the hyphen. 2 Co- with the hyphen is often used in compounds that are not yet standard (co-golfer), or to prevent ambiguity (co-driver—because codriver could be mistaken for cod river), or simply to avoid an awkward spelling (co-own is clearly preferable to coown). There are also some relatively less common terms, such as co-respondent (in a divorce suit), where the hyphenated spelling distinguishes the word’s meaning and pronunciation from that of the more common correspondent.