The difference between a lunar day and the rotational period of the Moon lies in the reference frame and the specific quantities they measure.
The rotational period of the Moon, also known as its "spin period" or "tidal locking period," refers to the time it takes for the Moon to complete one full rotation on its axis. The Moon is tidally locked to the Earth, which means that it takes the same amount of time for the Moon to complete one rotation as it does to complete one orbit around the Earth. Consequently, the Moon's rotational period is approximately equal to its orbital period, which is about 27.3 Earth days.
On the other hand, a lunar day refers to the time it takes for a specific location on the Moon to experience one complete cycle of day and night. Due to the Moon's tidally locked nature, the same side of the Moon always faces the Earth, while the opposite side is known as the "far side" or "dark side" of the Moon. As a result, the lunar day is equivalent to the Moon's orbital period, which is about 27.3 Earth days.
It's important to note that a lunar day does not refer to the time it takes for the Moon to complete one rotation on its axis. Since the Moon is tidally locked, observers on the Moon's surface experience one sunrise and one sunset per lunar day, but they don't observe the Moon spinning on its axis. This is why the lunar day is equivalent to the Moon's orbital period rather than its rotational period.
In summary, the rotational period of the Moon refers to the time it takes for the Moon to complete one rotation on its axis, which is approximately equal to its orbital period. A lunar day, however, refers to the time it takes for a specific location on the Moon to experience one complete cycle of day and night, and it is equivalent to the Moon's orbital period.