What is "transtage"

Transtage was an American upper stage used on Titan III rockets. Forty were launched, of which three failed.

The first launch, boosted by a Titan IIIA carrier rocket, occurred on 1 September 1964. The Transtage failed to pressurise, resulting in premature engine cutoff, and a failure to reach orbit. The second launch, on 10 December, was successful. The last launch of a transtage occurred on 4 September 1989, boosted by a Titan 34D rocket.