Tap cover.
Tap cover. A cap, usually made of plastic, that fits over the top of the tap (or bung) of a keg.
Source
27 CFR § 7.1
Scoping language
When used in this part and on forms prescribed under this part, the following terms have the meaning assigned to them in this section, unless the terms appear in a context that requires a different meaning. Any other term defined in the Federal Alcohol Administration Act (FAA Act) and used in this part has the same meaning assigned to it by the FAA Act.