A three-dimensional space enclosed by more than two thirds e/3) of the possible protected plane surface area and of sufficient size to allow the entry of personnel. For a typical building, this would require that 2/3 or more of the walls, ceiling, and/or floor are present.
Source: API RP 14G, Recommended Practice for Fire Prevention and Control on Fixed Open-type Offshore Production Platforms: Upstream Segment, Fourth Edition, April 2007. Global Standards
Any room or enclosure within which, in the absence of artificial ventilation, the ventilation will be limited and any flammable atmosphere will not be dispersed naturally.
Source: IEC 61892-7, Mobile and fixed offshore units – Electrical installations – Part 7: Hazardous areas. Global Standards