An inertial dampener (or inertial damper) was a technology which altered the inertia of matter. Inertial dampeners were vital components of warp vessels, allowing them to quickly change relativistic speeds without endangering crew or internal components. (ST reference: Star Trek Encyclopedia) A starship could not accelerate to warp speed without an active inertial dampener, as it would kill the entire crew instantly. (VOY episode: "Tattoo")