Key1  —n. (pl. -s) 1 (usu. Metal) instrument for moving the bolt of a lock. 2 similar implement for operating a switch. 3 instrument for grasping screws, nuts, etc., or for winding a clock etc. 4 (often in pl.) Finger-operated button or lever on a typewriter, piano, computer terminal, etc. 5 means of advance, access, etc. (key to success). 6 (attrib.) Essential (key element). 7 a solution or explanation. B word or system for solving a cipher or code. C explanatory list of symbols used in a map, table, etc. 8 mus. System of notes related to each other and based on a particular note (key of c major).