U-bend is a bend whose shape is the U shape, and it is also called siphon.
The word siphon is sometimes used to refer to a wide variety of devices that involve the flow of liquids through tubes. But in the English language today, the word siphon usually refers to a tube in an inverted U shape which causes a liquid to flow uphill, above the surface of the reservoir, without pumps, powered by the fall of the liquid as it flows down the tube under the pull of gravity, and is discharged at a level lower than the surface of the reservoir.
In practical siphons, atmospheric pressure pushes the liquid up the tube into the region of reduced pressure at the top of the tube. The reduced pressure is caused by liquid falling on the exit side. In the laboratory, some siphons have been demonstrated to work in a vacuum, indicating the tensile strength of the liquid is contributing to the operation of siphons at very low pressures.