A stream is any body of flowing fluid. The most familiar type of stream is made of water, although streams can also be made of air, lava, electricity, or any other fluid.

Jet streams, for instance, are cold, fast-moving winds that circulate high in the atmosphere. Lightning is a stream of electricity that circulates from cloud to cloud, from the cloud to the ground, or even from the ground to a cloud. Hydrothermal vents eject a stream of vent fluid into the ocean that surrounds them. Although vent fluid contains water, it is much, much hotter than the water surrounding it, and filled with materials from the Earth's crust, such as sulfur, zinc, and copper. Vent fluid is sometimes visible as white or black streams pouring from the vent.

Still, the most familiar type of stream is made of free-flowing water. These streams are fed by rain, melting snow and ice, and groundwater—the water that penetrates deeper into the Earth after the surface soil is completely soaked. Streams vary in size from tiny rills or streamlets, to larger brooks, creeks, and rivers. The term “stream” is often used interchangeably with “river,” though “stream” usually refers to a smaller body of water.

Streams take on different shapes depending on the landscape through which they flow. Cascades, or waterfalls, are formed when shallow water flows over and around large rocks. Normally, waterfalls are found in mountainous areas.

A stream containing large amounts of sediment lining its bottom and sides can change its shape and develop large curves called meanders. Sometimes, meanders are so wide that they eventually are cut off from the main stream. The U-shaped body of water left behind is called an oxbow lake.

Large amounts of sediment in a stream may cause it to split into many intertwined channels, called braided channels. Braided rivers are different from meandering streams. While meandering streams find a single new channel, braided streams flow over a series of wide, shallow channels.

Human engineering efforts can change the flow of a stream. For example, a meander might be straightened to improve a shipping channel.

Multiple streams can flow together so that one stream eventually carries water that originated from different parts of a large area. This area is known as the stream’s drainage basin, or watershed. The Amazon River in South America is the world’s largest stream and also has the world’s largest drainage basin. At more than 6.9 million square kilometers (2.3 million square miles), it is nearly as large as the entire country of Australia.