A town charter is a type of contract and charter that incorporates a settlement that is at least 25 parcels in size into a town, the minimum size of settlement for what Valyria calls a boroughs. A boroughs is an entity that has a fealty agreement with a count; rather than power and vassalage residing in a single person, as with a baron, a boroughs is a collective entity.
The town charter is a Valyrian government document that is a game mechanic. Applying a town charter to a qualifying settlement installs a settlement management table in the settlement, and installs the majority landholder as the Lord Mayor of the Boroughs. A town charter will likely need to be drafted by a character with the scribe skill, which Valyria will employ in the role of clerks.