Texture EnVironment unit. It's basically the GC and Wii equivelant of Direct3D. Direct3D has the advantage of being well understood by the development community and the shaders can be programmed to operate as needed by the developer. The TEV is a fixed function pipeline in that all effects are locked in their application and few development studios have really taken the time to learn how capable the unit truly is. Some developers barely even touch it and don't use any graphical shaders at all. It's why you see games of tremedous graphical difference on GC and Wii between certain developers.
The rEVOLution is not being televised