|Noah Webster's Dictionary|
(n.) The central or topmost stone of an arch. This in some styles is made different in size from the other voussoirs, or projects, or is decorated with carving.
Keystone (3 Occurrences)
Ephesians 2:20 Resting on the base of the Apostles and prophets, Christ Jesus himself being the chief keystone, (BBE)
1 Peter 2:6 Because it is said in the Writings, See, I am placing a keystone in Zion, of great and special value; and the man who has faith in him will not be put to shame. (BBE)
Zechariah 10:4 From him will come the keystone, from him the nail, from him the bow of war, from him will come every ruler; (BBE)