Prime Directive is a Role-Playing Game set in the Star Fleet Universe. The Star Fleet Universe is set from the game Star Fleet Battles (which was inspired by the original TV series, but does not follow the canon).
In Prime Directive, players are part of a "Prime Team," which is a Special Forces unit for the Federation in the Star Fleet Universe.
Prime Directive (First Edition) was originaly released in 1993. Since then there have been three different versions released: d20, d20 Modern, and GURPS.