Purebred.

Purebred. A term applicable to animals which are the progeny of known and registered ancestors of the same recognized breed and for which at least three generations of ancestry can be traced: Provided, however, That in the case of sheep registered on the basis of flocks, the term is applicable to animals of a recognized breed which originate in a flock for which available breeding data, as shown in the registry association's records, establish that the flock has been in existence at least ten years.

Source

9 CFR § 151.1

Scoping language

Words used in this part in the singular form shall be deemed to import the plural, and vice versa, as the case may demand. As used in this part, the following words, names, or terms shall have the meanings set forth in this section, unless otherwise clearly indicated by the context.