key

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A low offshore island or reef, especially in the Gulf of Mexico; a cay.
• noun A kilogram of marijuana, cocaine, or heroin.
• noun A notched and grooved, usually metal implement that is turned to open or close a lock.
• noun A similar implement or an electronic device used for opening, winding, or starting something.
• noun A device, such as a wedge or pin, inserted to lock together mechanical or structural parts.
• noun A keycard.
• noun A determining factor in accomplishing or achieving something.
• noun Something that provides access to or understanding of something else.
• noun A set of answers to a test.
• noun A table, gloss, or cipher containing correspondences, as for decoding or interpreting something.
• noun Computers A number used by a cryptographic algorithm to encrypt or decrypt data.
• noun Architecture The keystone in the crown of an arch.
• noun A button or lever that is depressed to operate a machine.
• noun A button that is depressed to cause a corresponding character or function to be typed or executed by a typewriter or to be accepted as input by a computer.
• noun Music A button or lever that is depressed with the finger to produce or modulate the sound of an instrument, such as a clarinet or piano.
• noun A tonal system consisting of seven tones in fixed relationship to a tonic, having a characteristic key signature and being the structural foundation of the bulk of Western music; tonality.
• noun The principal tonality of a work.
• noun The pitch of a voice or other sound.
• noun A characteristic tone or level of intensity, as of a speech or sales campaign. Often used in combination.
• noun Botany A samara.
• noun An outline of the distinguishing characteristics of a group of organisms, used as a guide in taxonomic identification.
• noun Basketball An area at each end of the court between the baseline and the foul line and including the jump-ball circle at the foul line.
• noun The act of replacing portions of a video or photograph containing a preselected color with material from a separate image, as by chroma key.
• adjective Of crucial importance; significant.
• intransitive verb To lock with a key.
• intransitive verb To be the determining or crucial factor in.
• intransitive verb Architecture To furnish (an arch) with a keystone.
• intransitive verb Music To regulate the pitch of.
• intransitive verb To bring into harmony; adjust or adapt.
• intransitive verb To cause to pay attention to.
• intransitive verb To supply with a key of correspondences or with corresponding references.
• intransitive verb To operate (a device), as for typesetting, by means of a keyboard.
• intransitive verb To enter (data) into a computer by means of a keyboard.
• intransitive verb To identify (a biological specimen).
• intransitive verb To vandalize or mar by scratching with a key.
• intransitive verb To produce, replace, or include by chroma key.
• intransitive verb To pay close attention; focus.
• intransitive verb Sports To watch or cover an opposing player closely in an effort to limit the player's effectiveness. Used with on:
• intransitive verb To replace portions of a video or photograph containing a preselected color with material from a separate image, as by chroma key.
• idiom (in key) In consonance with other factors.
• idiom (out of key) Not in consonance with other factors.

from The Century Dictionary.

• noun A wharf. See quay.
• noun An instrument for fastening or opening a lock, fitted to its wards, and adapted, on being inserted and turned or pushed in the keyhole, to push a bolt one way or the other, or to raise a catch or latch; in certain complicated locks, a portable appliance which on being inserted in the proper place in the lock lifts tumblers or in some other way allows the bolt to be shot without itself exercising force upon it.
• noun Hence Something regarded as analogous to a key, in being a means of opening or making clear what is closed or obscure; especially, that by means of which (often by means of which alone) some difficulty can be overcome, some obstacle removed, some end attained, something unintelligible explained, etc: as, the key to knowledge; Gibraltar is the key to the Mediterranean; a key to the solution of an algebraic problem; a key to an algebra or arithmetic (a book giving the solution of mathematical problems proposed as exercises in such text-books); the key to a cipher.