Confused, too!
I looked it up:
"Term of art"
A word or phrase used by legal professionals that has a precise meaning in a particular subject area. Knowing the terms of art and their definitions for your area of practice will save you time and effort in your research.
Examples of terms of art include:
"blue sky laws" (refers to state securities laws)
"TINA" (an acronym for the Truth in Negotiation Act)