Texture coordinate
From Tech Artists Wiki
A texture coordinate is a point in texture space- that is, it uses U, V, and W coordinates, as opposed to X, Y, and Z. A vertex can have an arbitrary amount of texture coordinates, since they exist in a unique space, but each texture coordinate belongs to only one vertex. It is important to note, however, that since shaders only process a single vertex-texture coordinate pair per UV channel, what appears as a single vertex with n texture coordinates in a 3D app will be broken into n vertices during rendering.
With 2D textures, the W coordinate is usually ignored- only the U and V are considered. For operations that only consider coordinates inside the 0-1 area, however, moving points in the W will move the point outside of the operation's effect (an example is baking normal maps).
