Occupancy is the act of owning, renting, or taking possession of a building. When you and your family move into a new house, you are taking occupancy of that home.
You're probably most familiar with the word occupancy from signs in auditoriums and concert halls that state the room's "maximum occupancy." A maximum occupancy of 100 means only 100 people can safely fit inside that room at one time. A single-occupancy hotel room is meant for one person. The opposite of occupancy is vacancy––when no one is in possession. Hotels let you know when they have a vacancy by flashing a sign.