Any of the cyclic deformations of one astronomical body caused by the gravitational forces exerted by others. The most familiar are the periodic variations in sea level on the Earth that correspond to changes in the relative positions of the Moon and the Sun. At the surface of the Earth the gravitational force of the Moon is about 2.2 times greater than that of the Sun. The tide-producing action of the Moon arises from the variations in its gravitational field over the surface of the Earth as compared with its strength at the Earth's centre. The effect is that the water tends to accumulate on the parts of the Earth's surface directly toward and directly opposite the Moon and to be depleted elsewhere. The regions of accumulation move over the surface as the position of the Moon varies relative to the Earth, mainly because of the Earth's rotation but also because of the Moon's orbital motion around the Earth. There are approximately two high and two low tides per day at any given place, but they occur at times that change from day to day; the average interval between consecutive high tides is 12 hours 25 minutes. The effect of the Sun is similar and additive to that of the Moon. Consequently, the tides of largest range or amplitude (spring tides) occur at New Moon, when the Moon and the Sun are in the same direction, and at Full Moon, when they are in opposite directions; the tides of smallest range (neap tides) occur at intermediate phases of the Moon.
Source: Encyclopaedia Britannica 2002