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A jet stream is a high-altitude, high-speed, west-to-east stream of air in Earth atmosphere. They are narrow (compared to the distance from equator to pole) and they tend meander (following trajectories that curve, northward and southward in turn), and their trajectories shift with changing weather. Typical characteristics:
Their latitude tends to be more poleward during summer and closer to the equator in winter. There are generally two jet streams in the northern hemisphere and two in the southern, located at the boundaries between the circulation cells (Hadley cell, Ferrel cell, and polar vortex), the poleward stream called the polar jet stream, and that nearer the equator called the subtropical jet stream. Wave-like shifts in the location of the border between the Ferrel cell and polar vortex (which the polar jet stream follows) are termed Rossby waves.
Other planets can have analogous jet streams, the term often shortened to jets. These are located at the borders demarcating the planet's bands, such as the east-west stripes visible on the surface of Jupiter and Saturn.
In astrophysics, the term jet is also used for directed flows of matter, typically sent outward along the axis-of-rotation of a black hole or pre-main-sequence star.