ETYM as. tîd time.
1. Something that may increase or decrease (like the tides of the sea).
2. The periodic rise and fall of the sea level under the gravitational pull of the moon.
3. There are usually two high and two low tides each day; SYN. lunar time period.
The rhythmic rise and fall of sea level in Earth's oceans and their inlets and estuaries due to the gravitational attraction of the Moon and, to a lesser extent, the Sun, affecting regions of Earth unequally as it rotates. Water on the side of Earth nearest the Moon feels the Moon's pull and accumulates directly below it producing high tide.
High tide occurs at an interval of 12 hr 24 min 30 sec. The maximum high tides, or spring tides, occur at or near new and full Moon when the Moon and Sun are in line and exert the greatest combined gravitational pull. Lower high tides, or neap tides, occur when the Moon is in its first or third quarter and the Moon and Sun are at right angles to each other.