**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:
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(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.
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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.
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