In common parlance, a batarang is a bat-shaped throwing weapon used by comic-book hero Batman. To make a somewhat obvious point, the name derives from a portmanteau of the words "bat" and "boomerang."
Originally spelled baterang, the batarang made its first appearance very early on in the Batman comics, in Detective Comics #31 (September 1939). As the name implies, the original batarang was a boomerang-style weapon which was thrown and then returned to the thrower. However, in recent incarnations of the Batman story, the batarang is generally a razor-sharp, metal, shuriken-like weapon that is much smaller than a boomerang and by no means returns to the thrower.
In addition to appearing in the comics, the batarang has appeared in every live-action and animated screen adaptation of Batman to date. However, somewhat in contrast to the batmobile, the batwing, and the batcave, the batarang is almost never referred to by name. This stems in part from the generalized deprecation of the "bat-" prefix since the excesses of the 1960s television show, but undoubtedly also because of the fact that the modern-day batarang does not resemble or behave like a boomerang.
Nevertheless, the "batarang" name lives on in everyday speech as the undisputed standard term for this weapon among Batman aficionados.