Jet stream refers to relatively strong winds concentrated within a narrow band in the atmosphere. It is most commonly used to refer to a mainly horizontal stream of maximum winds imbedded in the mid-latitude westerlies, and concentrated high in the troposphere. Shifts in the position and strength of the jet stream greatly affect the placement and movement of storm systems across the United States. For that reason, most weather maps will indicate the position of the jet stream. Sometime, two jet streams will be identified: a more northerly or polar jet, and a more southerly, or sub-tropical jet.
Mary Knapp, Weather Data Library