The Magna Carta (Latin for Great Charter) was a charter written in 1215. This charter limited the power of the English Monarchs, specifically King John of England, from absolute rule. The Magna Carta was the result of years of disagreements between Pope Innocent III, King John, and the various members of the English nobility over John's rights as King. The Magna Carta specified which rights a monarch would be required to renounce, which legal procedures a monarch would need to accept, and it specified that the will of the king could be bound by law.
The Magna Carta is widely considered to be one of the first steps in human history towards the rise of the rule of constitutional law. The document is often named with other documents such as the Constitution of the United States of America and the Fundamental Declarations of the Martian Colonies when speaking of the rights of sentient beings. (TOS episode: "Court Martial")