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c.1600, from Latin apex "summit, peak, tip, top, extreme end;" probably related to apere "to fasten, fix," hence "the tip of anything" (one of the meanings in Latin was "small rod at the top of the flamen's cap"), from PIE *ap- "to take, reach." Proper plural is apices.
apex a·pex (ā'pěks)
n. pl. a·pex·es or a·pi·ces (ā'pĭ-sēz', āp'ĭ-)
The pointed end of a conical or pyramidal structure.