The inertial dampening field was an energy field created by inertial dampers.
While piloting a shuttlecraft through a dark matter nebula, Captain Jean-Luc Picard inquired if it were possible to get a power boost transfer from the microfusion thrusters, Data informed his that it was, "if you augment the thruster sequencing so that the inertial dampening fields..." but was cut off before he could finish because the shuttle's inertial dampers had failed, and it was out of control. (TNG: "In Theory")