The Constitution, so "constitution" can be kept for an overview of constitutions in general.
Talk about it here.
The Constitution was a legal document of the Time Lords society on Gallifrey, set in articles. It could be amended on proposition of the Lord President and agreement of the High Council. (TV: The Deadly Assassin)
Long before the Doctor's departure from Gallifrey, one of the Lord Presidents rewrote the Constitution of the Time Lords to allow the Lord President to name his successor. His intention was to create a family dynasty. (PROSE: Birth of a Renegade)