What was Enterprise's NASA orbiter vehicle designation?

Enterprise’s NASA orbiter vehicle designation was OV-101.

Enterprise was NASA’s first Space Shuttle orbiter and was built as a test vehicle rather than a spaceworthy spacecraft. The designation means “Orbiter Vehicle 101.” It was used for ground tests, captive flights attached to a Boeing 747 Shuttle Carrier Aircraft, and free-flight approach-and-landing tests at Edwards Air Force Base in 1977.

Enterprise demonstrated that the shuttle orbiter could separate from its carrier, maneuver like a glider, and land safely on a runway. NASA originally planned to name the vehicle Constitution, but a major letter-writing campaign by Star Trek fans helped persuade President Gerald Ford to choose Enterprise instead.

Enterprise never reached orbit because it lacked important systems, including engines, orbital maneuvering hardware, and full thermal protection. NASA considered converting it for spaceflight, but the plan was abandoned because structural changes would have been too costly. OV-102 was Columbia, OV-099 was Challenger, and OV-105 was Endeavour.