Black's Law Dictionary, Sixth Edition, p. 1605
Words of art. The vocabulary or terminology of a particular
art, science, or profession, and especially those expressions which
are idiomatic or peculiar to it. For example, in law "Taking the
Fifth" means that a person is asserting his or her Fifth Amendment protection
against self-incrimination.
[Black's Law Dictionary,
Sixth Edition, p. 1605]