Length of One Day
The length of one (solar) day on a planet is the time from noon to noon. Since the planets spin on their axes at different rates and travel about the Sun at different speeds as well, the length of one day is different for each planet. For example, on Earth the length of a solar day is 24 hours. But because of its slow rotation and its rapid orbital speed, one solar day on Mercury lasts about as long as 176 days on Earth.
As an interesting side note, on Earth our solar day is very nearly the same as our sidereal day. Our solar day is 24 hours, which, generally speaking, is the length of time which elapses between the Sun reaching its highest point in the sky two consecutive times. Our sidereal day is 23 hours and 56 minutes, which is so close to our solar day that we often fail to distinguish between the two. A sidereal day is the length of time which passes between a given "fixed" star in the sky crossing a given projected meridian (line of longitude). However, on Mercury these two "days" are quite different! The time it takes for the Sun to reach the highest point in the sky two consecutive times on Mercury (the solar day) is 176 Earth days whereas a sidereal day on Mercury with respect to the stars is 59 Earth days.
How long is a day on Mercury? Watch the Sun rise and set on Mercury in the "A Day on Mercury" animation. Click on the image above.