The Prime Directive is one of the guiding principles of the UN, which is that James T. Kirk is authorized to interfere with any alien culture that his ship, the UNC Enterforaprise, runs across. This has often gotten Kirk in trouble with the Oil-for-Food Program when he either refuses to interfere with an alien culture or wrecks his ship. In fact, an entire Holovision series was made about this, entitled "Star Wreck."
Not to be confused with the Optimus Prime Directive, the governing body of the Autobots.