Etymology
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apex (n.)

"the tip, point, or summit" of anything, c. 1600, from Latin apex "summit, peak, tip, top, extreme end;" which is plausibly related to apere "to fasten, fix," hence "the tip of anything" (one of the meanings of apex in Latin was "small rod at the top of the flamen's cap"), and thus ultimately from PIE *ap- (1) "to take, reach" (see apt). But if the original notion was "point," not "top," it might go another way. Proper plural is apices.

updated on September 23, 2022

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Definitions of apex from WordNet

apex (n.)
the highest point (of something);
Synonyms: vertex / peak / acme
apex (n.)
the point on the celestial sphere toward which the sun and solar system appear to be moving relative to the fixed stars;
Synonyms: solar apex / apex of the sun's way
Etymologies are not definitions. From wordnet.princeton.edu, not affiliated with etymonline.