The Collaborative International Dictionary of English v.0.48:

Solstice \Sol"stice\, n.[L. solstitium; sol the sun + sistere to
   cause to stand, akin to stare to stand: cf. F. solstice. See
   Solar, a., Stand, v. i.]
   1. A stopping or standing still of the sun. [Obs.] --Sir T.
      Browne.
      [1913 Webster]

   2. (Astron.)
      (a) The point in the ecliptic at which the sun is farthest
          from the equator, north or south, namely, the first
          point of the sign Cancer and the first point of the
          sign Capricorn, the former being the summer solstice,
          latter the winter solstice, in northern latitudes; --
          so called because the sun then apparently stands still
          in its northward or southward motion.
      (b) The time of the sun's passing the solstices, or
          solstitial points, namely, about June 21 and December
          21. See Illust. in Appendix.
          [1913 Webster]

WordNet (r) 3.0 (2006):

solstice
    n 1: either of the two times of the year when the sun is at its
         greatest distance from the celestial equator