Inertia negation is a hypothetical process causing physical objects with mass to act as if they were of lower mass or were massless. The effect is the opposite of adding ballast, and does not exist in the real world. There is currently no known material or technology that is able to eliminate or negate the effects of inertia that all known types of mass possess.
According to Newton's first law, "A body will continue in its state of rest or of uniform motion in a straight line, unless compelled to change that state by a net force". Inertia is the resistance against changes in the motion of an object. Objects within objects each possess their own inertia, and will collide with each other when the containing object is moved.
A device that would be capable of inertia negation is described as being capable of reducing the inertia of both the larger containing object, and of all contained objects within, so as to make changes in motion easier, and to reduce or prevent damage due to internal collisions. The inertia is not absorbed or redirected but simply ceases to have a physical effect.
The results of using such a device may have significant unforeseeable consequences. If the device completely negates motion in space, the affected object or area would appear to move very fast to outside observers. This is in part because of Earth's rotation and its orbit around the sun, the rotation and velocity of the galaxy or an unmeasurable movement of the entire universe. In addition, it is impossible to determine how the object or area would be affected, if at all, by time dilation.
Antimatter, while being the opposite of matter, would still have inertia with the forces oriented in the same direction as normal matter. Storing antimatter on-board a vehicle made of matter would not counter the inertial effects of the mass that constitutes the vehicle.
Inertia negation is a commonplace technology in the fictional Star Trek universe, but are incorrectly described as inertia dampers, which are real devices but have a completely different purpose.
Inertia negation is used to counter the effects of sudden acceleration that would impart structural stresses on star ships when suddenly accelerating to or decelerating with the impulse drive, and which would cause passengers to be thrown against walls and crushed by the inertial effects of the vehicle suddenly accelerating or slowing.
Such device does not need to negate or alter inertia - a similar effect can be achieved by creating a gravitational field opposing the acceleration of the vessel. Such technology, while still nonexistent is by far more realistic than manipulating inertial mass.
Inertial dampers also appeared in later episodes of Stargate SG-1 and in the series Stargate Atlantis.