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.
• To fasten with a key, or with a wedge-shaped piece of wood or metal; fasten or secure firmly.
• To regulate the tone of by the use of a key, or to set to a key or pitch in any way, as a musical instrument: as, to key up a violin.