The annulus, or annular space, is the space between a penetrant and anything that surrounds it, such as the sides of an opening or a sleeve, depending on the case. It is named after the corresponding geometric concept due to its shape.
References
External links External examples of the use of the term. Annulus, a term used within a firestop patent Annulus, as used in UL System C-AJ-1557 Firestop System, Configuration A