The Dean drive is a device intended to be a reactionless thruster

Reactionless drive

A reactionless drive is a fictional or theorized method of propulsion where thrust is generated without any need for any outside force or net momentum exchange to produce linear motion...

 that was invented by Norman L. Dean. Dean claimed that it was able to generate a uni-directional force, in violation of Newton's Third Law of Motion

Newton's laws of motion

Newton's laws of motion are three physical laws that form the basis for classical mechanics. They describe the relationship between the forces acting on a body and its motion due to those forces...

. Such a violation is generally considered to be impossible in physics

Physics

Physics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...

. While it is theoretically possible for a mass that moves in one direction to have its momentum balanced by something other than a reaction mass (e.g. see Nuclear photonic rocket

Nuclear photonic rocket

In a nuclear photonic rocket, a nuclear reactor would generate such high temperatures that the blackbody radiation from the reactor would provide significant thrust. The disadvantage is that it takes a lot of power to generate a small amount of thrust this way, so acceleration is very slow. The...

), there is no known theoretical mechanism for a mass to be moved one way while nothing moves the other way, at least in a flat spacetime.

An actual functioning "reactionless thruster" would, of course, have enormous application, completely changing human transport, engineering, space travel and more.

According to Dean, his drive is a reactionless thruster, and his models were able to demonstrate this effect.