A shotgun is a smooth bore gun (not being an air-weapon) which:
Other smooth bore guns may require a firearms certificate or even be prohibited weapons.
There are weapons other than conventional shotguns which also require a shotgun certificate, e.g. smooth bore muskets used for re-enactments.
Genuine antique shotguns, with flintlock, matchlock and percussion cap firing mechanisms, which are not kept to be fired, will not normally require to be certificated.
If in doubt please contact your Firearms Licensing Department.