Anti-gravity is the concept of creating a force to counter the effects of gravity, typically in the context of spacecraft propulsion. Such systems are limited to the realm of science fiction given the current understanding of the way gravity works, but this has not stopped legions of hopefuls from making various spinning disks and magnets in hopes of perfecting such a device.
Gravitation was once thought to be a force between two objects. That is, something, gravity, creates a force on objects, causing them to move together. Under this Newtonian version of gravity it seems reasonable to suspect that it may be possible to create a system to interfere with this force, thereby lessening the effects of gravity on an object. Such a material, Cavorite, is a major deus ex machina of H. G. Wells' famous book, The First Men in the Moon.
However Newtonian gravitation is no longer thought to be correct. In 1915 Albert Einstein published his work on General Relativity which proposed an entirely new mechanism for gravity - one based entirely on the geometry of the universe. Gravity was no longer a force at all, but simply the consequense of the local slope of the universe in a direction the human eye cannot see: time. You stick to the floor not because gravity is pulling you down, but because that is the shortest distance between today and tomorrow. Under this model gravity is always attractive; one could, in theory, reduce the local curvature to zero and thus have no gravity, but there is no negative curvature corresponding to anti-gravity.
Not to be stopped, many anti-gravity aficionados have moved onto various quantum gravity theories in order to keep the dream alive. To date, none of these theories have proven fruitful.
