Proof.
Proof. The ethyl alcohol content of a liquid at 60 degrees Fahrenheit, stated as twice the percentage of ethyl alcohol by volume.
Source
27 CFR § 5.1
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.