expert witness
noun[C]
LAWukusapersonwho isaskedto givetheiropinionon aparticularsubjectin alawcourtbecause oftheirknowledgeorpracticalexperienceof thatsubject:
The industry'sexpertwitnesstestifiedthat thestatelawswouldincreasecarpricesby $5,000.
Heappearedas anexpertwitnessin a breach-of-contractcase.