A sol is the standard astronomical term for one solar day on Mars.
One Martian sol lasts about 24 hours, 39 minutes, and 35 seconds. It is the time Mars takes to rotate once relative to the Sun, so it is about 39 minutes longer than an average Earth solar day. Mission teams use sols to schedule rover operations, communications, and science observations.
A sol is not exactly the same as Mars’s sidereal rotation period, which measures rotation relative to distant stars. Mars’s sidereal day is about 24 hours 37 minutes 22 seconds; the slightly longer solar day results from Mars moving along its orbit while rotating.
The term is especially common in mission operations. NASA engineers and scientists may describe a rover’s activities as occurring on Sol 100 or Sol 500, counting from that mission’s landing date. A Martian year lasts about 687 Earth days, so seasonal planning requires a separate calendar from the sol-based daily schedule.