A jet stream is a narrow band of strong wind that generally blows from west to east all across the globe. Jet streams are relatively narrow bands of strong wind in the upper levels of the atmosphere, typically occurring around 30,000 feet (9,100 meters) in altitude. They are fast-flowing, meandering air currents in the atmospheres of the Earth, Venus, Jupiter, Saturn, Uranus, and Neptune. On Earth, the main jet streams are located near the altitude of the tropopause and are westerly winds, flowing west to east.

Jet streams can start, stop, split into two or more parts, combine into one stream, or flow in various directions, including opposite to the direction of the remainder of the jet. They are typically continuous over long distances, but discontinuities are also common. The path of the jet typically has a meandering shape, and these meanders themselves propagate eastward, at lower speeds than that of the actual wind within the flow. Each large meander, or wave, within the jet stream is known as a Rossby wave (planetary wave).

Meteorologists use the location of some of the jet streams as an aid in weather forecasting. The main commercial relevance of the jet streams is in air travel, as flight time can be dramatically affected by either flying with the flow or against it. Often, airlines work to fly with the jet stream to obtain significant fuel cost and time savings. Jet streams can also affect our weather by impacting temperature and precipitation. Some weather systems ride the jet stream across the country. Or, if the weather system is far away from a jet stream, it might stay in one place, causing a heat wave or a flood. Monitoring jet streams can help meteorologists determine where weather systems will move next.