A galaxy's tidal arm (or tidal tail) is a "tail" on a galaxy of gas and stars. Such a galaxy is called a tidal-tail galaxy. These are stars pulled out of the galaxy by a nearby galaxy by its galactic tide, a process called tidal stripping.
Such a tail is a sign of a galaxy merger, e.g., if no other galaxy is near enough to produce it. Such a merger can instigate star formation, including within the tail: on the order of a tenth of such a galaxy's star formation can be within the tail.
The term tidal stream is used for similar structures drawn by tidal forces from clouds or stellar clusters.