key

– noun
1. a small metal instrument specially cut to fit into a lock and move its bolt.
2. something that affords a means of access.
3. something that secures or controls entrance to a place.
4. something that affords a means of clarifying a problem.
5. a book, pamphlet, or other text containing the solutions or translations of material given elsewhere, as testing exercises.
6. a systematic explanation of abbreviations, symbols, etc., used in a dictionary, map, etc.
7. the system, method, pattern, etc., used to decode or decipher a cryptogram, as a code book, or machine setting.
8. one of a set of marked parts, designated areas, or levers pressed in operating a typewriter, computer terminal, calculator, etc.
9. a manually operated lever for opening and closing an electric circuit, used to produce signals in telegraphy.
10. a. one of the levers that when depressed by the performer sets in motion the playing mechanism.
b. a metal lever that opens and closes a vent.
c. the relationship perceived between all tones in a given unit of music and a single tone or a keynote; tonality.
d. the principal tonality of a composition.
e. the keynote or tonic of a scale.
11. tone or pitch, as of voice.
12. mood or characteristic style, as of expression or thought.
13. degree of intensity, as of feeling or action.
14. a pin, bolt, wedge, or other piece inserted in a hole or space to lock or hold parts of a mechanism or structure together; a cotter.
15. a small piece of steel fitting into matching slots of a hub of a wheel or the like and the shaft on which the wheel is mounted so that torque is transmitted from one to the other.
16. a contrivance for grasping and turning a bolt, nut, etc.
17. a field or group of characters within a record that identifies the record, establishing its position among sorted records, and/or provides information about its contents.
18. a unique code inserted for each medium used, to determine the relative effectiveness of the media.
19. the dominant tonal value of a picture.
20. the tonal value and intensity of a color or range of colors
21. chief; major; important; essential; fundamental; pivotal.
– verb
22. to regulate or adjust to a particular state or activity; bring into conformity.