In the context of astronomy: strongly focussed, highly energetic particle streams like those emitted by certain active galactic nuclei. Jets can become visible when they deposit their energy in a huge gaseous region, making them shine out bright (so-called radio bubbles).
The unit of energy in the International System of units (SI). One Joule is equal to one kilogram times square metre over square second, in short: 1 J = 1 kg m²/s². It is equal to the kinetic energy gained by an object of the mass one kilogram that has been accelerated with an acceleration of one metre per square second over a distance of one metre.