Webster's English Dictionary

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key

1. key \'ke-\ n [ME, fr. OE c-g; akin to MLG keige spear] 1a: a usu. 
   metal instrument by which the bolt of a lock is turned  1b: any of various 
   devices having the form or function of such a key  2: a means of gaining or 
   preventing entrance, possession, or control  3a: something that gives an 
   explanation or provides a solution  3b: a list of words or phrases giving 
   an explanation of symbols or abbrevia tions 3c: an arrangement of the 
   salient characters of a group of plants or animal s or of taxa designed to 
   facilitate identification 3d: a map legend  4a1: COTTER PIN  4a2: COTTER  
   4b: a keystone in an arch  4c: a wedge used to make a dovetail joint  4d: a 
   small parallel-sided piece that fits into a groove and prevents relat ive 
   motion between rotating parts; also : a wedge for drawing or holding parts 
   together 5a: one of the levers of a keyboard musical instrument that 
   actuates the me chanism and produces the tones 5b: a lever that controls a 
   vent in the side of a woodwind instrument or a  valve in a brass instrument 
   5c: a depressible digital that serves as one unit of a keyboard and that wo 
   rks usu. by lever action to set in motion a character or an escapement (as 
   in some typesetting machines) 6: SAMARA  7: a leading individual or 
   principle  8: a system of seven tones based on their relationship to a 
   tonic; speci f : the tonality of a scale 9a: characteristic style or tone : 
   STRAIN  9b: the tone or pitch of a voice  9c: the predominant tone of a 
   photograph with respect to its lightness or d arkness 10: a decoration or 
   charm resembling a key  11: a small switch for opening or closing an 
   electric circuit 
2. key vt 1: to lock with a key : as  1a: to secure (as a pulley on a 
   shaft) by a key  1b: to finish off (an arch) by inserting a keystone  2: to 
   regulate the musical pitch of  3: to make conformable : ATTUNE  4: to 
   identify (a biological specimen) by a key  5: to insert in (an 
   advertisement) matter intended to identify answers  6: to make nervous or 
   tense - usu. used with up  : to use a key 
3. key aj : of basic importance : FUNDAMENTAL 
4. key n [Sp cayo, fr. Lucayo] : a low island or reef; specif : one of the 
   coral islets off the s outhern coast of Florida