Retired was a title used by retired military officers to indicate that they were no longer active members of the military service.
After the USS Enterprise encountered the alien Trelane in 2267, Captain James T. Kirk referred to him by his rank of general, and each time, Trelane reminded him he was "Retired, (sir)." (TOS: "The Squire of Gothos")
Following an instance of espionage aboard the USS Enterprise-D, the investigation was conducted by the visiting retired Admiral Norah Satie. (TNG: "The Drumhead")
When Admiral Jean-Luc Picard attempted to persuade Captain Liam Shaw to divert the USS Titan-A to the Ryton system, Picard reminded Shaw that "Respectfully, Captain, I am an admiral," but Shaw quickly added, "Retired. Congrats on that." (PIC: "The Next Generation")