(plural sidereal days)
Although the two above definitions are identical in everyday practice, a sidereal day is normally measured relative to the point of vernal equinox. This, combined with the effect that precession has on the vernal equinox, makes a sidereal day 8 milliseconds shorter than the actual rotational period. However, this difference is small enough that it is usually ignored, so the term sidereal day is also used for the time of a full 360 degree rotation. When the difference is significant, the term stellar day is used to refer to the time of a full rotation, which is measured relative to the stars.
sidereal day - Computer Definition
Slightly less than the 24 hours of a solar day (23 hours, 56 minutes, 4.1 seconds). Used in astronomy, sidereal means "when a star crosses the meridian" (the meridian is an imaginary circle around the earth). For example, at noon, the sun and stars are overhead, but 23 hours, 56 minutes and 4.1 seconds later, the earth is back in relation to the stars, but not the sun, which takes approximately four minutes longer.