What is Confined Space?

Legal Definition
A confined space is an enclosed area with limited space and accessibility. An example is the interior of a storage tank, which may be occasionally entered by workers for maintenance but is otherwise not a habitable space. Hazards in a confined space often include harmful dust or gases, asphyxiation, submersion in liquids or free-flowing granular solids (for example, grain bins), electrocution, or entrapment.

Confined space accidents are of particular concern in occupational safety and health due to the hazards that they pose to the victim and subsequently to a rescue team. Confined space training outlines the skills and protocols for safe entry to confined spaces, and includes precautions such as locking and tagging out connecting piping, testing of breathable air quality, forced ventilation, observation of workers in the space, and a predetermined rescue plan with appropriate safety harnesses and other rescue equipment standing by.
-- Wikipedia
Legal Definition
Dangerous area with small openings making an exit difficult. Lack of ventilation and hazardous chemical can be very dangerous. It is not meant to have a person in this place for a long time. Lack of oxygen and fumes can trap the person.