Tide, the rhythmic rising and falling of the surface of the oceans, seas, and other bodies of water. (Similar rhythmic movements, also called tides, occur in the earth's crust and atmosphere, but these movements can be detected only with sensitive scientific instruments.)

When the water flows in toward the land, it is at flood tide; the highest level it reaches is high tide. The water recedes during ebb tide; the lowest level it reaches is called low tide. The part of the shore between the high-tide line and the low-tide line is the intertidal zone. Here live plants and animals adapted to survive both on land and underwater. Barnacles, kelp, and some forms of crabs and clams live in the intertidal zone.

In most areas, there are two high tides and two low tides in approximately a day. Tidal movement occurs over the entire area of any large body of water. It is noticeable, however, only where the water's rise and fall can be measured against the land.

Tides have been carefully observed because of their importance in navigation. Most of the world's harbors are affected by noticeable tides. The depth of submerged hazards, the amount of water in channels, and the direction of the current, which all depend at least in part on the tide, affect the safety of ships passing into and out of the harbors.

Tides vary greatly from place to place around the world. Tide predictions for a specific area must therefore be based on an extensive series of observations and measurements in that area. Even so, tide predictions are not completely accurate because variable factors, such as weather conditions, have a measurable effect on the tides. Tidal information is published in the newspapers of coastal cities, in almanacs, and in tide tables used aboard ships.

The rising and falling water of the tides possesses great kinetic energy (the energy of motion). Part of this energy is expended against the shores. The erosion of shorelines is due in part to the tides. In a few countries, including France and Canada, the tides have been used to generate electricity. .)

In some rivers the tide produces a tidal bore. Tidal waves are not in any way related to tides. These destructive waves are caused by undersea earthquakes and volcanic eruptions or by storms at sea.