The Belle Reve was a small, state-of-the-art Starfleet starship disguised to look like a commercial freighter for use in covert missions with which the United Federation of Planets and Starfleet can not be openly associated. It has a Defiant-class warp core and heavy armaments, spartan crew quarters, and a bridge with three forward viewscreens.
The ship was previously named the Calypso, but its name was changed when it was permanently assigned to Captain James T. Kirk in 2380. In addition to the name change, when he was given formal command of the vessel, Kirk ordered the ship undergo a refit to make it more comfortable for its crew. (TOS novel: Captain's Blood)