Jet Cooling System – A jet pump is used to produce a vacuum so water or refrigerant may evaporate at relatively low temperatures. These systems usually require large condenser and have a low efficiency to remove heat. 

Joint - Connecting point as between two pipes. 

Joule- Metric unit of heat. 

Joule –Thomson Effect -  The change in the temperature of a gas on its expansion through a porous plug from a higher pressure to a lower pressure. 

Journal Crankshaft -  The part of shaft which contacts the bearing on the large end of the piston rod. 

Junction Box – A box or container housing group of electrical terminals.