Repoussé is the name of the new 3D features in Photoshop CS5 Extended. The word repoussé is actually a French term that refers to a process of forming a relief into a sheet of metal or something by hammering a shape from one side. How it came to be the name of this feature is a long story but basically it refers to Photoshop ability to use internal constraints to push and pull textures and surfaces in 3D. Though it is only part of what this feature can do the name somehow stuck.
Another point to remember is that Repoussé along with all the other 3D features are only available in Photoshop CS5 Extended version.