key¹ (kē)

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[Middle English kai, kei, from Old English cǣg.]

noun: pl., keys. 

1. 1. A notched and grooved, usually metal implement that is turned to open or close a lock.
   2. A similar device used for opening or winding: the key of a clock; a can that has a key attached.
2. A means of access, control, or possession.
3. 1. A vital, crucial element.
   2. A set of answers to a test.
   3. A table, gloss, or cipher for decoding or interpreting.
4. A device, such as a wedge or pin, inserted to lock together mechanical or structural parts.
5. Architecture The keystone in the crown of an arch.
6. 1. A button or lever that is depressed to operate a machine.
   2. 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.
   3. 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.
7. Music
   1. 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.
   2. The principal tonality of a work: an etude in the key of E.
8. The pitch of a voice or other sound.
9. A characteristic tone or level of intensity, as of a speech or sales campaign. Often used in combination: high-key; low-key.
10. Botany A samara.
11. An outline of the distinguishing characteristics of a group of organisms, used as a guide in taxonomic identification.
12. Basketball An area at each end of the court between the base line and the foul line and including the jump-ball circle at the foul line: a jump shot from the top of the key.

adjective 

Of crucial importance; significant: key decisions; the key element of the thesis.

verb: keyed, key·ing, keys. 

transitive verb 

1. To lock with or as if with a key.
2. Architecture To furnish (an arch) with a keystone.
3. Music To regulate the pitch of.
4. To bring into harmony; adjust or adapt.
5. To supply an explanatory key for.
6. 1. To operate (a device), as for typesetting, by means of a keyboard.
   2. To enter (data) into a computer by means of a keyboard.
7. To identify (a biological specimen).
8. To vandalize or mar by scratching with a key: Vandals keyed the cars left in the parking garage.

intransitive verb 

1. To pay close attention; focus: improved service by keying on customer complaints; keyed in on the main points of the lecture.
2. Sports To watch or cover an opposing player closely in an effort to limit the player's effectiveness. Used with on: “ still carries the burden of scoring … even though opponents key on her throughout every game” (Josh Barr)

phrasal verbs

key up

To make intense, excited, or nervous.

idioms

in key

In consonance with other factors.

out of key

Not in consonance with other factors.

key² (kē)

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[Alteration (influenced by key) (variant of quay), of Spanish cayo; see cay.]

noun: pl., keys. 

A low offshore island or reef, especially in the Gulf of Mexico; a cay.

key³ (kē)

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[Shortening and alteration of kilogram.]

noun: pl., keys. 

A kilogram of marijuana, cocaine, or heroin.