The time required for a single rotation of the earth on its axis in reference to any star or to the vernal equinox at the meridian, equal to 23 hours, 56 minutes, 4.09 seconds in units of mean solar time.
The time between two successive passages of the vernal equinox across the meridian: it measures one rotation of the earth and equals 23 hours, 56 minutes, 4.1 seconds of mean solar time.
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.