A Jet (or Polar Jet) is a stream of matter (e.g., Plasma) sent with apparent force from an astronomical body such as a Black Hole. Their mechanism is not agreed upon, but some theories incorporate magnetism. Bodies with Accretion Disks such as Protostars as well as active galaxies often show jets called "polar jets" sent in one or both directions along the axis of rotation. In such cases, they may be referred to as Disk Wind. They are called Relativistic Jets when the matter is emitted at speeds approaching light speed.
In the case of stars, high spin makes them oblate, and the surface at the poles is nearer the core, thus hotter, and jets can form.
The word jet is also in the phrase Jet Stream, which is sometimes shortened to jet.