[jcranmer]

There are two different definitions of "day". The sidereal day is the time it takes for a body to complete a full 360° rotation about its axis. The synodic day is the time between success zenith passings of the parent body. If the body rotates in the same direction as its orbits, the synodic day is longer than the sidereal day; if its rotation is in the opposite direction, the sidereal day is longer.

On Earth, the synodic day is 24 hours long, and the sidereal day is about 4 minutes shorter. In the case of Mercury, the sidereal day is 59 Earth days and the synodic day is 176 Earth days. Note that the synodic day is the one we on Earth use to define the length of a day for timekeeping purposes--it's a lot easier to measure synodic time than sidereal time.