uniform acceleration. The speeding up of an object in which the velocity increases at a uniform rate. It changes the same amount each second it moves.
uniform surface corrosion. A mild form of corrosion in which the surface of a piece of metal is covered by the salts of corrosion, but there are no localized pits on the surface.
unijunction transistor (UJT). A three-terminal semiconductor device. A pulse of energy on one terminal controls the flow of electrons between its other two terminals. When a pulse of electrical energy of the correct polarity is applied between the emitter and one of the bases, a negative resistance is produced between the two bases, and electrons flow through the UJT between the two bases.

union (threaded pipe fitting). A three-piece threaded pipe fitting used to join two pipes or to connect a pipe to a component. A union allows the pipes to be connected without either of the pipes having to be turned.
United States. 14 CFR Part 1: “In a geographical sense means: (1) the States, the district of Columbia, Puerto Rico, and the possessions, including the territorial waters, and (2) the airspace of those areas.”
United States air carrier. 14 CFR Part 1: “A citizen of the United States who undertakes directly by lease, or other arrangement, to engage in air transportation.”
unit magnetic pole. A magnetic pole that will repel a like pole of equal strength, at a distance of one centimeter, in a vacuum, with a force of one dyne.
universal chuck (machine tool component). A type of clamp used to hold material being turned on a lathe. The three jaws of a universal chuck are moved in and out at the same time by turning a chuck key. A universal chuck holds the work centered as the jaws are clamped tightly on the work.