1. (Also called tidal day.) The time required for the earth to rotate once with respect to the moon, that is, the time between two successive upper transits of the moon. The mean lunar day is approximately 1.035 times as great as the mean solar day, or 24 hours 50 minutes. 2. In astronomy, the time required for the moon to revolve once, relative to a fixed star, about its own axis.
In space exploration, a lunar day is the period of time it takes for the Moon to complete one full rotation on its axis with respect to the Sun. Equivalently, it is the time it takes the Moon to make one complete orbit around the Earth and come back to the same phase. It is marked from a New Moon to the next New Moon.