n. A prefix of Latin origin, the usual form, before a vowel or h, of com- (the m in Latin being weak), meaning ‘together’ or ‘with.’ See com-.n. In geometry, a prefix, as in co-sine, co-secant, co-tangent, etc., meaning sine, secant, tangent, etc., of the complement.n. An abbreviation of company: as, Smith, Brown & Co.;n. of county: as, Orange Co., New York.n. The chemical symbol for cobalt.