[ter.ti.um quid] n [LL, lit., third something; fr. its failing to fit into a dichotomy] (ca. 1724) 1: a middle course or an intermediate component "where there are two systems of law and two orders of courts, there must ... be some tertium quid to deal with conflicts of law and jurisdiction --Ernest Baker"
2: a third party of ambiguous status "there was a man and his wife and a tertium quid --Rudyard Kipling"