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
See sidereal day in American Heritage Dictionary 4
noun
The time required for a complete rotation of the earth 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.