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From The Collaborative International Dictionary of English v.0.48:
Vertex \Ver"tex\, n.; pl. E. Vertexes, L. Vertices. [L. vertex, -icis, a whirl, top of the head, top, summit, from vertere to turn. See Verse, and cf. Vortex.] A turning point; the principal or highest point; top; summit; crown; apex. Specifically: [1913 Webster] (a) (Anat.) The top, or crown, of the head. [1913 Webster] (b) (Astron.) The zenith, or the point of the heavens directly overhead. [1913 Webster] (c) (Math.) 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. [1913 Webster] Note: The principal vertex of a conic section is, in the parabola, the vertex of the axis of the curve: in the ellipse, either extremity of either axis, but usually the left-hand vertex of the transverse axis; in the hyperbola, either vertex, but usually the right-hand vertex of the transverse axis. [1913 Webster] Vertex of a curve (Math.), the point in which the axis of the curve intersects it. Vertex of an angle (Math.), the point in which the sides of the angle meet. Vertex of a solid, or Vertex of a surface of revolution (Math.), the point in which the axis pierces the surface. [1913 Webster]