- (astronomy) one day of sidereal time; the time in which Earth rotates once around its axis, relative to the point of the vernal equinox (about 23 hours, 56 minutes, 4 seconds)
- (astronomy) rotational period of Earth; the time in which Earth rotates around its axis 360 degrees
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.