The Collaborative International Dictionary of English v.0.48:

Sidereal \Si*de"re*al\, a. [L. sidereus, from sidus, sideris, a
   constellation, a star. Cf. Sideral, Consider, Desire.]
   1. Relating to the stars; starry; astral; as, sidereal
      astronomy.
      [1913 Webster]

   2. (Astron.) Measuring by the apparent motion of the stars;
      designated, marked out, or accompanied, by a return to the
      same position in respect to the stars; as, the sidereal
      revolution of a planet; a sidereal day.
      [1913 Webster]

   Sidereal clock, day, month, year. See under Clock,
      Day, etc.

   Sideral time, time as reckoned by sideral days, or, taking
      the sidereal day as the unit, the time elapsed since a
      transit of the vernal equinox, reckoned in parts of a
      sidereal day. This is, strictly, apparent sidereal time,
      mean sidereal time being reckoned from the transit, not of
      the true, but of the mean, equinoctial point.
      [1913 Webster]