Interstellar Law is a series of enacted rules intended to govern the relationships between sovereign planetary and interplanetary governments. It is generally developed through multilateral accords, conventions and other legal instruments agreed to and generally enforced by major powers, such as the Klingon Empire, the Federation and the Cardassian Union.
In 2293 Captain Kirk was charged with the assassination of Klingon Chancellor Gorkon, a crime under interstellar law. It was later discovered that the Chancellor was murdered by participants in the Khitomer conspiracy, and Kirk was cleared of all charges. (Star Trek VI: The Undiscovered Country)
Interstellar Law was a major offered at the Oneamisu Campus of the University of Hurkos. The core focus of this major was the study of Interstellar Law. Upon completion of this major, the student received a Bachelor of Arts. In the late 2340s, Devinoni Ral switched from this major to Political Science. (TNG: "The Price")
As a matter of interstellar law, Bajor was not recognized as a separate political entity during the Cardassian Occupation. It was only formally recognized following the end of the Occupation in 2369. (DS9: "Emissary")