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 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.