Domestic Terrorism has already been defined in law. (18 U.S. Code § 2331 - Definitions)

 As used in this chapter (click here) - 

(1)the term “international terrorism” means activities that—

(A)

involve violent acts or acts dangerous to human life that are a violation of the criminal laws of the United States or of any State, or that would be a criminal violation if committed within the jurisdiction of the United States or of any State;

(B)appear to be intended—

(i)

to intimidate or coerce a civilian population;

(ii)

to influence the policy of a government by intimidation or coercion; or

(iii)

to affect the conduct of a government by mass destruction, assassination, or kidnapping; and

(C)

occur primarily outside the territorial jurisdiction of the United States, or transcend national boundaries in terms of the means by which they are accomplished, the persons they appear intended to intimidate or coerce, or the locale in which their perpetrators operate or seek asylum;

(2)

the term “national of the United States” has the meaning given such term in section 101(a)(22) of the Immigration and Nationality Act;

(3)

the term “person” means any individual or entity capable of holding a legal or beneficial interest in property;

(4)the term “act of war” means any act occurring in the course of—

(A)

declared war;

(B)

armed conflict, whether or not war has been declared, between two or more nations; or

(C)

armed conflict between military forces of any origin;

(5)the term “domestic terrorism” means activities that—

(A)

involve acts dangerous to human life that are a violation of the criminal laws of the United States or of any State;

(B)appear to be intended—

(i)

to intimidate or coerce a civilian population;

(ii)

to influence the policy of a government by intimidation or coercion; or

(iii)

to affect the conduct of a government by mass destruction, assassination, or kidnapping; and

(C)

occur primarily within the territorial jurisdiction of the United States; and

(6)the term “military force” does not include any person that—

(A)has been designated as a—

(i)

foreign terrorist organization by the Secretary of State under section 219 of the Immigration and Nationality Act (8 U.S.C. 1189); or

(ii)

specially designated global terrorist (as such term is defined in section 594.310 of title 31, Code of Federal Regulations) by the Secretary of State or the Secretary of the Treasury; or

(B)

has been determined by the court to not be a “military force”.