Noun (Fem. 1st Decl.)
Perseus Dictionary Entry (Liddell and Scott [and Jones]'s Greek-English Lexicon, 9th ed., 1925-1940)
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LXX uses κεφαλὴν γωνίας to translate לְרֹ֣אשׁ פִּנָּֽה (capstone [or keystone?]) in Psalm 118:22
(cf. ἀκρογωνιαῖον for פִּנַּ֤ת in Isaiah 28:16: ἀκρογωνιαῖος):
λίθον ὃν ἀπεδοκίμασαν οἱ οἰκοδομοῦντες οὗτος ἐγενήθη εἰς κεφαλὴν γωνίας (Psalm 117:22 LXX)
The stone the builders rejected / has become the capstone (NIV).
A capstone or coping stone is one of the finishing or protective stones that form the top of an exterior masonry wall or building (http://en.wikipedia.org/wiki/Capstone).
A keystone is the architectural piece at the crown of a vault or arch and marks its apex, locking the other pieces into position (http://en.wikipedia.org/wiki/Keystone_(architecture)).
Occurrences in the GNT
Occurrences in the LXX