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 § 19.1
Scoping language
As used in this part, the following terms shall have the meanings indicated unless either the context in which they are used requires a different meaning, or a different definition is prescribed for a particular subpart, section, or portion of this part: