Any enclosed (or largely enclosed) place where there is a reasonably foreseeable risk of fire, explosion, loss of consciousness, asphyxiation or drowning counts as a 'confined space' for health and safety purposes. Note that a space doesn't necessarily have to be small, difficult to enter or completely enclosed to count as 'confined'.
The Health and Safety Executive (HSE) uses the above definition when explaining which workplaces are covered by confined space safety rules.
