What is the name of the protocol droid in Star Wars?

The protocol droid in Star Wars is C-3PO.

Pronounced “See-Threepio,” C-3PO is a humanoid protocol droid designed for etiquette, translation and diplomatic assistance. He is famous for claiming fluency in more than six million forms of communication, making him useful in situations involving unfamiliar species, governments and machines.

The prequel trilogy reveals that the young Anakin Skywalker built C-3PO on Tatooine from spare parts. C-3PO later becomes closely associated with R2-D2, and the pair serve alongside characters including Luke Skywalker, Leia Organa and Han Solo. Their contrasting personalities—C-3PO's anxious formality and R2-D2's resourceful silence—make them one of the franchise's defining partnerships.

A common mix-up is choosing R2-D2, who is also a droid but belongs to the astromech class and specializes in navigation, repairs and computer systems. BB-8 is another astromech, while IG-88 is an assassin droid. C-3PO is the protocol droid.