Black's Law Dictionary, Sixth Edition, p. 423:

definition.    A description of a thing by its properties; an explanation of the meaning of a word or term.  The process of stating the exact  meaning of a word by means of other words.  Such a description of the thing defined, including all essential elements and excluding all nonessential, as to distinguish it from all other things and classes."

[Black's Law Dictionary, Sixth Edition, p. 423]

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Eddie Kahn: 

Because the term "includes" is defined expansively in 26 U.S.C. Sec. 7701(c), any "definition" that uses this word is a NON definition and cannot be relied upon to clearly and unambiguously establish the meaning of a word.  Any definition that uses "means" instead of "includes", however, is a legitimate definition that does properly bound the meaning of a word.  You will note that 26 U.S.C. Sec. 7701 has a mixture of definitions, some of which use the word "means" and others use the word "includes".  Don't pay any attention to the definitions that use the word "includes" because they are just double speak deliberately designed to confuse you.  This kind of double speak is evident, for instance, in the definition of the term "United States" found in 26 U.S.C. Sec. 7701(a)9, and represents a violation of due process, since it doesn't clearly define the jurisdiction of the federal government to tax because it uses the word "includes" instead of "means".

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Great IRS Hoax, Section 5.3.3, Version 4.50:

In the event that definitions of such terms as “U.S.**” or “includes” has you confused about your tax obligations or the applicability of the above table, remember that the supreme court said of the rules of statutory construction as found in the case of Hassett v. Welch, 303 U.S. 303:

“In view of other settled rules of statutory construction, which  teach that... if doubt exists as to the construction of a taxing statute,  the doubt should be resolved in favor of the taxpayer...”

The reason for this is obvious.  The Sixth Amendment says that we have a right to due process and to know and understand the charges against us.  How can we be assured of a fair trial or justice if we can't definitively even know or limit what the law says in such a way  that we can completely understand what is expected of us and obey it?  Furthermore, the definition of the word "definition" found in Black's Law Dictionary, Sixth Edition, page 423:

"A description of a thing by its properties; an explanation of the meaning of a word or term.  The process of stating the exact   meaning of a word by means of other words.  Such a description of the thing defined, including all essential elements and excluding all nonessential, as to distinguish it from all other things and classes."

If the term "includes" found in 26 U.S.C. Sec. 7701(c) is used "expansively",  NOTHING is defined in the Internal Revenue Code ANYWHERE! For such a case the whole code is "Void for Vagueness" as described in section 5.12!

[Great IRS Hoax, Section 5.3.3, Version 4.50]