Imperial salute
From Wookieepedia, the Star Wars wiki.
The Imperial salute was a form of greeting used within COMPNOR and, to a lesser extent, within the Imperial military. A soldier using the Imperial salute stood at attention and held out his right arm with the lower arm raised in a 45 degrees angle. The hand was held open, showing the palm. This gesture was typically accompanied by the phrases "Hail the Empire!", "Hail the Emperor!", or some variant thereof.
[edit] Behind the scenes
The Imperial salute is rather obviously based on the Hitler salute used in Nazi Germany.
