A periapical abscess is a localized, purulent form of periapical periodontitis.
The abscess can track through the bone to the soft tissue creating a localized fluctuant swelling, normally adjacent to the affected tooth root. The abscess will then fistulize and drain or spread to surrounding tissues causing cellulitis.
Sutherland S, Matthews DC. Emergency management of acute apical periodontitis in the permanent dentition: a systematic review of the literature. Journal Canadian Dental Association. 2003;69(3):160.