A sanctum is any place used as a headquarters, living quarters, or work space by one or more Mages. Sanctums can take a variety of forms and serve a range of functions, depending on the needs of the resident cabal (or cabals). They typically boast magical fortifications, but that's not always the case.
The common image of a sanctum is that of a moderately sized Victorian house, but that paints a very limited picture. A sanctum can be anywhere: A monastery, a cave, an underground tunnel system, an industrial park. Any place a cabal of mages resides is, by definition, a sanctum. Ideally, a sanctum is located on a Hallow in order to provide Mana for the resident mages, as well as to fuel any long-term spells on the structure itself. In the modern day, the increasing rarity of Hallows makes it difficult to construct new sanctums atop them, although masters of the Prime Arcanum have been known to use powerful magic to move an existing Hallow to a newly built sanctum.
A more personalized variation of the Sanctum is the Demesne.