Material Unaccounted For, a term from the nuclear industry.
Nuclear reactors come in two flavors: those intended for producing electricity, and those intended only to make more fissile material, either U-235 or plutonium, for use in other reactors or in nuclear weapons. Some electricity-producing reactors are also breeder reactors, which means that they produce more fuel than they consume. In its efforts to keep tabs on which nations are nuclear powers and which ones are not, The International Atomic Energy Agency tries to keep track of all nuclear materials in the world. Reactor employees predict how much fissile material should be produced by the nuclear reactions taking place, and then the material itself is isolated periodically and reported to the IAEA. Any disparity between the prediction and reality is MUF.
MUF gets interesting when less material is recovered than is predicted, because then there can be allegations that people are stealing fissile material for the construction of illicit weapons. According to the Tom Clancy novel The Sum of All Fears, Israel's nuclear arsenal was built using MUF from various American reactors. Using techno-thrillers as historical sources is highly specious, however; a node on how Israel's arsenal came to be is in order.