Sol (borrowed from the Latin word for sun) is a solar day on Mars; that is, a Mars-day.
A sol is the apparent interval between two successive returns of the Sun to the same meridian (sundial time) as seen by an observer on Mars.
The sol was adopted in 1976 during the Viking Lander missions and is a measure of time mainly used by NASA when, for example, scheduling the use of a Mars rover.
[5] When a NASA spacecraft lander begins operations on Mars, the passing Martian days (sols) have been tracked using a simple numerical count.
[9] NASA planners coined the term soliday at least as far back as 2012 to refer to days off due to time phasing or the syncing of planetary schedules.