The Prime Directive was a Federation, as well as Lyran, Starfleet directive that prohibited Starfleet from interfering in the internal affairs of any civilization, especially pre-warp civilizations, lest the contact disrupt the natural development of the civilization. (TOS: "Bread and Circuses", et al.)
In 2410, Kanril Eleya cited the Prime Directive, along with Federation laws against cloaking device research, as reasons for not giving said technology to the Klingon-Cardassian Alliance. However, she reasoned that giving the Alliance techniques to defeat Terran Starfleet cloaks did not violate the law. (Bait and Switch: The Wrong Reflection)
In 2411, Kanril Eleya intervened to stop a nuclear war on the planet Volante that her science division had projected as an extinction-level event. Though she initially sought permission to intervene from Starfleet Command, she acted without orders when the war began before she had been given authorization. She later justified herself to Admiral Eric Velasquez that the Prime Directive had come into conflict with General Order 24: to wit, the deliberate depopulation of an inhabited planet could only be enacted on an authenticated order from the President of the Federation and the Federation Council, and her deliberate inaction would have led to such an outcome. (Bait and Switch: "Shortest War Ever")
The Lyran Starfleet Prime Directive was somewhat less restrictive than the Federation counterpart; when recovering sensitive equipment or personnel, such as prisoners of war, Starfleet personnel could interfere with internal affairs. (Star Trek: The Stoneship Files: "Alcoholic Massacre, Part I")
Original Series producer Gene L. Coon is credited with the invention of the Prime Directive, but is said to have meant for it to be applied only to viable, pre-warp civilizations.