Frequently mentioned in Scripture. It is called in Hebrew maphteah, i.e., the opener (Judges 3:25); and in the Greek New Testament kleis, from its use in shutting (Matthew 16:19; Luke 11:52; Revelation 1:18, etc.). Figures of ancient Egyptian keys are frequently found on the monuments, also of Assyrian locks and keys of wood, and of a large size (Comp. Isaiah 22:22).

The word is used figuratively of power or authority or office (Isaiah 22:22; Revelation 3:7; Revelation 1:8; Comp. 9:1; 20:1; Comp. also Matthew 16:19; 18:18). The "key of knowledge" (Luke 11:52; Comp. Matthew 23:13) is the means of attaining the knowledge regarding the kingdom of God. The "power of the keys" is a phrase in general use to denote the extent of ecclesiastical authority.

Noah Webster's New International Dictionary of the English Language

1. (n.) An instrument by means of which the bolt of a lock is shot or drawn; usually, a removable metal instrument fitted to the mechanism of a particular lock and operated by turning in its place.

2. (n.) An instrument which is turned like a key in fastening or adjusting any mechanism; as, a watch key; a bed key, etc.

3. (n.) That part of an instrument or machine which serves as the means of operating it; as, a telegraph key; the keys of a pianoforte, or of a typewriter.

4. (n.) A position or condition which affords entrance, control, pr possession, etc.; as, the key of a line of defense; the key of a country; the key of a political situation. Hence, that which serves to unlock, open, discover, or solve something unknown or difficult; as, the key to a riddle; the key to a problem.

5. (n.) That part of a mechanism which serves to lock up, make fast, or adjust to position.

6. (n.) A piece of wood used as a wedge.

7. (n.) The last board of a floor when laid down.

8. (n.) A keystone.

9. (n.) That part of the plastering which is forced through between the laths and holds the rest in place.

10. (n.) A wedge to unite two or more pieces, or adjust their relative position; a cotter; a forelock.

11. (n.) A bar, pin or wedge, to secure a crank, pulley, coupling, etc., upon a shaft, and prevent relative turning; sometimes holding by friction alone, but more frequently by its resistance to shearing, being usually embedded partly in the shaft and partly in the crank, pulley, etc.

12. (n.) An indehiscent, one-seeded fruit furnished with a wing, as the fruit of the ash and maple; a samara; -- called also key fruit.

13. (n.) A family of tones whose regular members are called diatonic tones, and named key tone (or tonic) or one (or eight), mediant or three, dominant or five, subdominant or four, submediant or six, supertonic or two, and subtonic or seven. Chromatic tones are temporary members of a key, under such names as sharp four, flat seven, etc. Scales and tunes of every variety are made from the tones of a key.

14. (n.) The fundamental tone of a movement to which its modulations are referred, and with which it generally begins and ends; keynote.

15. (n.) Fig: The general pitch or tone of a sentence or utterance.

16. (v. t.) To fasten or secure firmly; to fasten or tighten with keys or wedges.