apex (n.)

"the tip, point, or summit" of anything, 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- (1) "to take, reach" (see apt). Proper plural is apices.