A droid built to assist in communications between humans, aliens and machines.
2000, Elizabeth B. Davis, Myth and Identity in the Epic of Imperial Spain, University of Missouri Press, ISBN 0-8262-1277-8, page 216,
Perhaps it is no accident that one of the most poignant moments in the whole Star Wars trilogy is the scene in Return of the Jedi where the protocol droid C-3PO relates the story of the Galactic Civil War for the assembled Ewoks in a language only they can understand.
2002, Pamela Rice Hahn and Jesse Flores, Journey to the Center of the Internet, Syngress, ISBN 192899475X, page 59,
“But moving right along, let’s talk about protocols,” Dr. F. continued.
Finally, a movie reference I understood! Finally, something I could get excited about! “Like C-3PO!” I shouted. “He was a protocol droid!”