The SS Defiant was an early Earth starship, sister ship to SS Valiant. Both ships were launched in 2064 on 20-year missions to find the limits of human reach with an improved warp drive. Defiant was lost a year after its departure.
It was later discovered that Defiant had been lost in an area of space claimed by the Barrier Alliance. (Star Fleet Starship Recognition Manual: Bonaventure Cruiser)