The Magna Carta was a historical legal document from Earth history. Laws pertaining to the Human rights of the accused were enumerated in this document.
It was one of multiple legal authorities cited by attorney at law Samuel T. Cogley at Captain Kirk's general court martial in 2267. (TOS: "Court Martial")
"Magna Carta" was Latin for "Great Charter". It is also seen spelled occasionally as "Magna Charta", although this wasn't the spelling used in the script of "Court Martial".
According to the Star Trek Encyclopedia, 4th ed., vol. 2, p. 11, this "great charter' of liberties [was] signed by English King John under enormous pressure from English barons on Earth in the year 1215. This document provided guarantees of due process in trials, and strict limitations of governmental power over the governed."