Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 n. The highest point; the apex or summit: the vertex of a mountain.
 n. Anatomy The highest point of the skull.
 n. Anatomy The top of the head.
 n. Astronomy The highest point reached in the apparent motion of a celestial body.
 n. Mathematics The point at which the sides of an angle intersect.
 n. Mathematics The point on a triangle or pyramid opposite to and farthest away from its base.
 n. Mathematics A point on a polyhedron common to three or more sides.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 n. The highest point of something.
 n. The highest surface on the skull.
 n. The common point of the two rays of the angle, or its equivalent structure in polyedra (meeting of edges) and higher order polytopes.
 n. A point on the curve with a local minimum or maximum of curvature.
 n. One of the elements of a graph joined or not by edges to other vertices.
 n. A point in 3D space, usually given in terms of its Cartesian coordinates.
 n. The point where the surface of a lens crosses the optical axis.
 n. An interaction point.
 n. The point where the prime vertical meets the ecliptic in the western hemisphere of a natal chart.
from the GNU version of the Collaborative International Dictionary of English
 n. A turning point; the principal or highest point; top; summit; crown; apex.
 n. The top, or crown, of the head.
 n. The zenith, or the point of the heavens directly overhead.
 n. 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.
from The Century Dictionary and Cyclopedia
 n. In geometry, the point at which concurrent straights meet.
 n. The highest or principal point; apex; top; crown; summit.
 n. In mathematics, a point of a figure most distant from the center; any convex angle of a polygon.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 n. the point of intersection of lines or the point opposite the base of a figure
 n. the highest point (of something)
Etymologies
Examples

Thus: what we call the vertex of the Brain is really its base and what we call its base is really its vertex, it is simply a question of nomenclature.

Thus the interior angle at each vertex is either 90° or 270°.

"What we call the vertex of the Brain is really its base: and what we call its base is really its vertex: it is simply a question of nomenclature."

The Harvard Medical School Family Health Guide, DHT promotes both the proliferation of prostate cells and hair loss, and that men who were diagnosed with prostate cancer were more likely to have had bald spots on the tops of their heads, which is referred to as vertex baldness, as opposed to those men who did not have bald spots.

At the vertex is a medical student named Karlanner (played by Stephen Barker Turner), a kind of emblematic conflicted "good German"  leftishly inclined, living with the Jewish girlfriend who rescued him from alcoholism.

The brightest star will be identified as the vertex of a rightangled triangle, of which the line from Vega to the Pole Star is the base, as shown in Fig. 91.

These last move more and more quickly until they reach what is called the vertex of the parabola (the point of such a path which lies nearest to the sun): projectiles, on the contrary, move more and more slowly as they approach the corresponding point of their path; and further, the comet first approaches and then recedes from the centre of attraction  the projectile first recedes from and then approaches the attracting centre.

Another is the ability to separate two components called vertex shaders and fragment shaders that previously had to be linked together.

Another is a feature DirectX programmers already enjoy, the ability to separate two components called vertex shaders and fragment shaders that previously had to be linked together.

For this parabola, a = 10 so it opens down and the vertex is the maximum point.
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fbharjo commented on the word vertex
a virtual texan??
December 11, 2010