Texture Mapping Unit (TMU) - A low-level GPU component that operates with some independence, entirely dedicated to manipulating bitmaps and texture filtration. TMUs modify bitmaps (resize, rotate, scale, skew, fit) for placement onto objects and filter textures; in video games, this would be represented as placing a texture onto an object, whereupon we have now created "texels" (textured pixels). Textured pixels require a unique approach to graphics processing, as explained in our "What is Texture Fill-Rate?" article.
Not all textures handled by a TMU are strictly "textures" as we know them in games, textures mapped by a TMU can also include light maps and bump maps. NVIDIA TMUs on Maxwell architecture can process 1 filtered pixel per unit, so a GTX 980 can filter 128 pixels per clock cycle. The GM204 Maxwell GPU advertises a texture filter rate of 144.1GT/s (or gigatexels per second), a TMU count of 128, and a core clock of 1126MHz. The formula can be found on our Texture Fill-Rate definition page. The GM204 GPU filters 128 texels per clock (Int & FP16), or 1 texel per TMU per clock.
AMD GPUs also function this way, but do not have equal INT & FP16 values. A Hawaii GPU filters 176 texels per clock cycle (INT) and just 88 texels per clock cycle (FP16), so depending on the task at hand and type of filtration, the theoretical max texture fill rate will vary on this hardware.