In the USA a confined space is defined as a relatively small or restricted space such as a tank, boiler, pressure vessel or small compartment. Confinement implies poor ventilation as a result of construction, size, or shape rather than restriction of egress of personnel.
Entry into a confined space must be justified, and needs special planning, to ensure that the person entering is not put at risk, and that if an unexpected event does arise, adequate rescue arrangements are in place. It is probable that a formal permit-to-work system will be required.
(UK) Confined Spaces Regulations 1997
(USA) ANSI Z117.1 'Safety requirements for confined spaces', or the pertaining OSHA standard.