A current is the rate of fluid flow, especially water in a river or ocean. A current can be any continuous, directed movement of a fluid.

In a river or stream, the water is influenced by gravity, and flows downhill to reduce its potential energy. The movement of water in a stream is called the current and this movement will vary from place to place and time to time dependent upon the volume of water, the slope, and the shape and other characteristics of the stream bed.

Ocean currents can also be caused by gravity as a result of the tide. Other ocean currents are caused by or influenced by rotation of the earth or differing densities of bodies of seawater resulting from differences in salinity or temperature—warmer water or less saline water is less dense and rises, whereas cooler water or more saline water is more dense and sinks.