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
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,
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.