Results
Definition of vertex:
 A turning point; the principal or highest point; top; summit; crown; apex.
 The point in any figure opposite to, and farthest from, the base; the terminating point of some particular line or lines in a figure or a curve; the top, or the point opposite the base.
 The top, or crown, of the head.
 The zenith, or the point of the heavens directly overhead.
Synonyms:
flush, vizor, extremum, bloom, summit, apex of the sun's way, height, top, zenith, heyday, tiptop, high, point, meridian, superlative, crown, pinnacle, prime, peak, cap, bill, flower, blossom, solar apex, visor, acme, elevation, crest, apex, roof, eyeshade, tip, efflorescence.
Usage examples:

The main branches run towards the vertex, and incisions should, as far as possible, be directed parallel with them.
 "Manual of Surgery Volume Second: ExtremitiesHeadNeck. Sixth Edition.", Alexander Miles Alexis Thomson. 
In the spring of 1852 a triangular mass, the vertex of which was just beyond or south of the Terrapin Tower, while its altitude of more than forty feet lay along the shore of the south corner of Goat Island, fell in the night with the usual grinding crash.
 "The Falls of Niagara and Other Famous Cataracts", George W. Holley. 
We have also the fact that " a triangle whose base is the diameter of a semicircle and whose vertex lies on the semicircle is a right triangle."
 "The Teaching of Geometry", David Eugene Smith.