The fire area is a building construction concept that allows the use of passive fire protection (i.e., fire-rated construction) instead of active fire protection (i.e., fire sprinklers). Its primary purpose is the protection of the structure and life safety is secondary. The NFPA 101 is focused primarily on life safety with a heavy emphasis on means of egress. It is not intended to be a building code--that is where the NFPA 5000 comes in.
NFPA 101 does not provide a definition for fire area, but NFPA 5000 does, which gives you an idea if a term is used within an NFPA code or standard. NFPA will define all applicable terms used within a code or standard in that code or standard. Unlike the NFPA, ICC states that terms not defined within a code but defined in other codes will have the same meaning as those defined in other codes.