com- repr. L. com-, arch. form of the prep. cum with, used in comps. with the meanings ‘together, in combination or union’, ‘altogether, completely’. Com- was retained before b, p, m, and some vowels (as in comes COUNT 1), assim. before r, l (as in corruptus CORRUPT, collātiō COLLATION), and reduced to CO- before most vowels; elsewhere it became CON- (but before f, as in COMFIT, COMFORT, com- has replaced con- in Eng.).

com- (co-, con-) From the Latin cum or com-, meaning ‘with’, a prefix meaning ‘with’, or ‘jointly’. ‘Com-’ is generally used before words beginning with b, m, and p, and sometimes before words beginning with vowels; ‘con-’ before words beginning with other consonants; and ‘co-’ before most words beginning with a vowel and before words beginning with h, gn, l, and r.