key 1 
(kē)
(kē)n. pl. keys
1.
a. A notched and grooved, usually metal implement that is turned to open or close a lock.
b. A similar implement or an electronic device used for opening, winding, or starting something: the key of a wind-up alarm clock; the new car's electronic key.
c. A device, such as a wedge or pin, inserted to lock together mechanical or structural parts.
d. A keycard.
2. A determining factor in accomplishing or achieving something: One key to the store's success has been consistent customer service.
3.
a. Something that provides access to or understanding of something else: The key to the mystery was a drug store receipt.
b. A set of answers to a test.
c. A table, gloss, or cipher containing correspondences, as for decoding or interpreting something.
d. Computers A number used by a cryptographic algorithm to encrypt or decrypt data.
4. Architecture The keystone in the crown of an arch.
5.
a. A button or lever that is depressed to operate a machine.
b. 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.
c. 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.
6. Music
a. 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.
b. The principal tonality of a work: an etude in the key of E.
7. The pitch of a voice or other sound.
8. A characteristic tone or level of intensity, as of a speech or sales campaign. Often used in combination: high-key; low-key.
9. Botany A samara.
10. An outline of the distinguishing characteristics of a group of organisms, used as a guide in taxonomic identification.
11. 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: a jump shot from the top of the key.
12. The act of replacing portions of a video or photograph containing a preselected color with material from a separate image, as by chroma key.
adj.
Of crucial importance; significant: key decisions; the key element of the thesis.
v. keyed, key·ing, keys
v.tr.
1. To lock with a key.
2. To be the determining or crucial factor in: a double that keyed a three-run rally in the fifth inning.
3. Architecture To furnish (an arch) with a keystone.
4. Music To regulate the pitch of.
5. To bring into harmony; adjust or adapt: "achievement tests that are more clearly keyed to what students are held responsible for in high school" (New York Times).
6. To cause to pay attention to: school officials who were keyed into the dietary needs of students.
7. To supply with a key of correspondences or with corresponding references: keyed the pages in the edited book to illustrations in the manuscript.
8.
a. To operate (a device), as for typesetting, by means of a keyboard.
b. To enter (data) into a computer by means of a keyboard.
9. To identify (a biological specimen).
10. To vandalize or mar by scratching with a key: Vandals keyed the cars left in the parking garage.
11. To produce, replace, or include by chroma key.
v.intr.
Phrasal Verb: 1. To pay close attention; focus: improved service by keying on customer complaints; keyed into 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: "[She] still carries the burden of scoring ... even though opponents key on her throughout every game" (Josh Barr).
3. To replace portions of a video or photograph containing a preselected color with material from a separate image, as by chroma key.
key up
Idioms: To make intense, excited, or nervous: The actors were keyed up before the opening.
in key
In consonance with other factors.
out of key
Not in consonance with other factors.
[Middle English kai, kei, from Old English cǣg.]