vertex of an angle

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.
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   (b) (Astron.) The zenith, or the point of the heavens
       directly overhead.
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   (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.
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   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]
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