The Oxford Thesaurus An A-Z Dictionary of Synonyms - key n.

1) latchkey, skeleton key, passkey, opener: Have you lost your car keys again? You will need a key to open the air valve on the radiator. 2) clue, cue, guide, indication, indicator, explanation: The key to his behaviour can probably be found in the way his mother treated him in his childhood. 3) pitch, tone, timbre, level, tonality, frequency: The song was originally written in the key of C. 4) legend, explanation, description, explication, clarification, translation: A key to the symbols appears on every other page. 5) mood, tenor, tone, humour, style: In this passage there is a change to a more sombre key. --adj. 6) important, essential, vital, necessary, crucial, critical, main, pivotal: Automatic reversal is a key feature of this cassette player. Roderick is a key man in the company.