What is a certificate of occupancy?

The Building Department of your local government issues a Certificate of Occupancy after inspecting a building and agreeing that it is safe to inhabit.

It is a document issued by a municipality certifying that a building's construction complies with all requirements for its purpose. If you're talking about a house then the municipality is certifying that the house is habitable.

I'm assuming the certificate from the fire marshall that states that a fire marshall has inspected your building and has calculated the maximum occupancy of the establishment.

Interesting answers. All correct, I believe.

A "certificate of occupancy" says that a building has been built to all applicable codes for it's purpose. The code enforcement official can revoke a "certificate of occupancy" at any time, if they find work done that compromises the safety and welfare of the occupants.

There are others like the maximum capacity for an elevator or the maximum number of people a given space can accommodate, but it still meets the above definition except restrictions have been posted.