The Collaborative International Dictionary of English v.0.48:

Equator \E*qua"tor\, n. [L. aequator one who equalizes: cf. F.
   ['e]quateur equator. See Equate.]
   1. (Geog.) The imaginary great circle on the earth's surface,
      everywhere equally distant from the two poles, and
      dividing the earth's surface into two hemispheres.
      [1913 Webster]

   2. (Astron.) The great circle of the celestial sphere,
      coincident with the plane of the earth's equator; -- so
      called because when the sun is in it, the days and nights
      are of equal length; hence called also the equinoctial,
      and on maps, globes, etc., the equinoctial line.
      [1913 Webster]

   Equator of the sun or Equator of a planet (Astron.), the
      great circle whose plane passes through through the center
      of the body, and is perpendicular to its axis of
      revolution.

   Magnetic equator. See Aclinic.
      [1913 Webster]

WordNet (r) 3.0 (2006):

equator
    n 1: an imaginary line around the Earth forming the great circle
         that is equidistant from the north and south poles; "the
         equator is the boundary between the northern and southern
         hemispheres"
    2: a circle dividing a sphere or other surface into two usually
       equal and symmetrical parts