Main  —adj. 1 chief, principal. 2 exerted to the full (by main force). —n. 1 principal duct etc. For water, sewage, etc. 2 (usu. In pl.; prec. By the) a central distribution network for electricity, gas, water, etc. B domestic electricity supply as distinct from batteries.  3 poet. High seas (spanish main).  in the main mostly. [old english]