The time taken to complete one rotation around a planet’s axis.
A sidereal day is not quite the same thing as a regular ‘day’.
Imagine standing on some point on the Earth and staring directly at a distant star. Keep perfectly still and wait for the Earth to complete one revolution about its axis. What will you be staring at now? It won’t be that same star again because during the time it took the Earth to rotate on its axis, it also moved along its orbit around the Sun. In fact the Earth will be about two and a half million kilometres away from where it was. So you’ll need to move your head slightly to see the star. Or wait a little longer for the Earth to rotate a little bit more so that you are again pointing in the same direction, staring at the star.
So if a regular ‘day’ were based only on the time it takes the Earth to complete one revolution about its axis, the Sun would reach its highest point in the sky at a different time each day. In other words, midday wouldn’t always be at noon.
So what we call a ‘day’ is technically known as a solar day. This is how long it takes for the Sun to move from being directly overhead (midday) to setting, to rising again, to being directly overhead again.
(Unfortunately this time period varies during the year because the orbit of the Earth is not a perfect circle, which means that the speed of its orbit around the Sun varies. So we base our timekeeping on an average, known as mean solar time.)
The diagram below illustrates the difference between a sidereal day and a solar day.
In position 1, the observer is looking directly to the left. In position 2, the Earth has rotated once about its own axis and again the observer is looking directly to the left. The time taken is one sidereal day.
However, in position 2, the observer is no longer looking at the Sun. He or she has to wait a little bit longer for the Earth to rotate a little more to make up for the fact the Earth has moved along in its orbit. A solar day is the time taken to get to position 3. Clearly this is slightly more than a sidereal day.