What is "muniments"

Usage examples of "muniments".

Then he reluctantly puttered into the muniments room, where the superefficient librarian had already got out the Stonehouse dossiers for him and put them on a table by the open doors, and sat down to read.

Stonehouse picks up a sheaf of paper from the desk: Argyll imagined it being part of the carefully handwritten inventory still in the Buonaterra muniments room.