From the building codes, Habitable Space: A space in a building for living, sleeping, eating or cooking. Bathrooms, toilet rooms, closets, halls, storage or utility spaces & similar areas are not considered habitable spaces.
Hope this helps
NEC does not define habitable space and does not refer us to another publication for any definition either. So we must go to the general definition of the word as used in general language when applying that word to the NEC.
Article 100 SCOPE, says the NEC does not define terms that are in general use by other codes.