Proof gallon. A gallon of liquid at 60 F. which contains 50 percent by volume of ethyl alcohol having a specific gravity of 0.7939 at 60 F. referred to water at 60 F. as unity, or the alcoholic equivalent thereof.
27 CFR § 5.11
When used in this part and in forms prescribed under this part, where not otherwise distinctly expressed or manifestly incompatible with the intent thereof, terms shall have the meaning ascribed in this section. Any other term defined in the Federal Alcohol Administration Act and used in this part shall have the same meaning assigned to it by such Act.