What was the name of the robot from Lost in Space?

Robot

Just to nitpick . . . in the second season episode "Ghost Planet" he is referred to as "Robot B-9." In every other episode he is addressed merely as "Robot."

In an episode where they travel back in time to just before they launch, there is a crate in Dr. Smiths office. It contains the Robot. On the side there is an acronym...G.U.N.T.H.E.R...which stood for General Utility Non Theorizing Environmental Robot. But as others have said, they always referred to him as Robot.