Relevant excerpts (although I encourage the interested reader to look
it up themselves):
18 USC 2510:
(4) "intercept" means the aural or other acquisition of the contents
of any wire, electronic, or oral communication through the use of any
electronic, mechanical, or other device.
[...]
(12) "electronic communication" means any transfer of signs, signals,
writing, images, sounds, data, or intelligence of any nature
transmitted in whole or in part by a wire, radio, electromagnetic,
photoelectronic or photooptical system that affects interstate or
foreign commerce, but does not include? [...]
18 USC 2511:
Except as otherwise specifically provided in this chapter any person who?
(a) intentionally intercepts, endeavors to intercept, or procures any
other person to intercept or endeavor to intercept, any wire, oral, or
electronic communication;
[...]
shall be punished as provided in subsection (4) or shall be subject to
suit as provided in subsection (5).
[...]
(4)
(a) Except as provided in paragraph (b) of this subsection or in
subsection (5), whoever violates subsection (1) of this section shall
be fined under this title or imprisoned not more than five years, or
both.