Major Rakal was the alias assumed by Deanna Troi when she was involuntarily recruited by the Romulan underground movement in 2369. Her cover story was that she was an agent of the Tal Shiar who was on a mission to the Kaleb sector, where she would deliver a cargo to the Corvallens.
Under this cover story, Troi was actually enabling the defection of Vice-Proconsul M'ret and his aides to the Federation.
The alias was based on a real Major Rakal who had been murdered by the Romulan Underground. (TNG: "Face of the Enemy")
As Rakal, Troi wore a unique "harness" with one additional strap over her Romulan uniform. It was never used again.