Cavity
Approved document B, Fire Safety, Volume 2, Buildings other than dwellinghouses (2019 edition), defines a cavity as:
A space enclosed by elements of a building (including a suspended ceiling) or contained within an element, but that is not a room, cupboard, circulation space, protected shaft, or space within a flue, chute, duct, pipe or conduit.
And a cavity barrier as:
A construction within a cavity, other than a smoke curtain, to perform either of the following functions.
- Close a cavity to stop smoke or flame entering.
- Restrict the movement of smoke or flame within a cavity.
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