An injection of polymer melt into the air, instead of into the mold. The main purpose of an air shot is to act as a "test run", to make sure that the machine is operating properly.
The cylinder is pulled back from contact with the sprue bushing and the injection piston is moved forward, forcing melted plastic from the nozzle. If the melt is caught in a metal cup, a pyrometer probe can be used to check the temperature of the melt as it exits from the nozzle. This is one of the more accurate measurements of the melt temperature.