Source : Webster's Revised Unabridged Dictionary (1913)

Vertex \Ver"tex\, n.; pl. {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:
   (a) (Anat.) The top, or crown, of the head.
   (b) (Anat.) The zenith, or the point of the heavens directly
       overhead.
   (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.

   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.

   {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 {of a surface of revolution} (Math.),
      the point in which the axis pierces the surface.