In the Supreme Court Case of Eisner v Macomber the court made the following quote concerning the power of the Congress to legislate Constitutional terms.
“Congress cannot by any definition it may adopt conclude the matter, since it cannot by legislation alter the Constitution, from which alone it derives its power to legislate, and within whose limitations alone that power can be lawfully exercised.”
The definition of gross income under the IRC code starts out as follows; Sec. 61. Gross income defined
-STATUTE-
(a) General definition
Except as otherwise provided in this subtitle, gross income means
all income from whatever source derived, including (but not limited
to) the following items:
It defines items included as income without defining income. Obey the law of the court.