On the subject of taxation, back in January one here finally found "taxpayer" defined in the Internal Revenue Manual while researching foreign estate EIN information.

21.7.13.2.1 (10-01-2012)
3.
Note:
  • A taxpayer is a Form 1040, U.S. Individual Income Tax Return, filer and has a Keogh plan, or is required to file excise, employment, or alcohol, tobacco, or firearms returns.