Plain English Meaning
How long Earth takes to spin around once when measured against a far-away star, rather than against the Sun. It is about four minutes shorter than the 24-hour day we use on the clock.
Full Definition
The time it takes Earth to complete one full rotation on its axis relative to a fixed point in space (a distant star), equal to approximately 23 hours, 56 minutes, and 4 seconds.
Context Anchor
Seen in GPS and navigation discussions when explaining why satellite patterns over Earth can repeat on a star-based daily cycle instead of exactly every 24 clock hours.
Derivation
From the Latin sidereus, meaning 'of the stars.' A sidereal day is measured against the stars, which is what makes it different from a solar day measured against the Sun.
Why Pilots Care
GPS satellite periods are set to sidereal days so their consistent with Earth's rotation, directly affecting signal and availability.