Relatively strong winds concentrated within a narrow band in the atmosphere.
WRCC Climate and Weather Terms Glossary

Rivers of high-speed air in the atmosphere. Jet streams form along the boundaries of global air masses where there is a significant difference in atmospheric temperature. The jet streams may be several hundred miles across and 1-2 miles deep at an altitude of 8-12 miles. They generally move west to east, and are strongest in the winter with core wind speeds as high as 250 mph. Changes in the jet stream indicate changes in the motion of the atmosphere and weather.
NASA Earth Observatory

Jet Stream - The major atmospheric air currents centers over the Pacific and Atlantic Oceans, North America and over the Middle East. These systems are fairly robust from period to period, and across different observational data sets. The most important jet stream from our perspective at Sam Houston State University, in Huntsville Texas, USA, is the Pacific Jet, which helps set the storm tracks over the west coast of North America--like most northern hemispheric weather--moving towards the east. Atmospheric Chemistry Glossary