The shield inverter is part of the deflector dish aboard Federation starships like the USS Enterprise-D.
By activating the shield inverter, the direction in which the deflector shield works cane be reversed, a pull-movement can be turned into a push-movement.
The shield inverter of the Enterprise-D was activated in 2368 after the deflector dish was used to absorb high-energy plasma from the atmosphere of Penthara IV. The plasma was released into space after activating the inverter. (TNG: "A Matter of Time")