Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   {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.

Source : WordNet®

sidereal
     adj 1: of or relating to the stars or constellations; "sidereal
            bodies"; "the sidereal system"
     2: (of divisions of time) determined by daily motion of the
        stars; "sidereal time" [ant: {civil}]