What does Green Belt (not to be confused with the term 'greenfield') mean in a planning context?

A designation for land around certain cities and large built-up areas, which aims to keep this land permanently open or largely undeveloped. The purposes of the green belt is to: check the unrestricted sprawl of large built up areas prevent neighbouring towns from merging safeguard the countryside from encroachment preserve the setting and special character of historic towns assist urban regeneration by encouraging the recycling of derelict and other urban land Green Belts are defined in a local planning authority’s development plan.