A transporter component that allows the exact state of every subatomic particle in a subject to be recorded, getting around the uncertainty principle that would otherwise make transport impossible.
Heisenberg's uncertainty principle says the position and momentum of a particle cannot both be known exactly, which ought to rule out copying a person atom by atom. The writers of The Next Generation answered the objection by giving the transporter a Heisenberg compensator. Asked how it works, technical adviser Michael Okuda famously replied that it works very well.
The component is named on screen in Realm of Fear, when Miles O'Brien and Reginald Barclay run checks on the transporter systems of the Enterprise-D.
In Ship in a Bottle, Data, La Forge and Barclay try to beam a holodeck object into the real world by uncoupling the compensators, part of the attempt to free the self-aware hologram Professor Moriarty.