ˌtərshēəmˈkwid, ˌtərd.ē-, ˌterd.ē- noun
Etymology: Late Latin, literally, third something, translation of Greek triton ti
1. : a middle course or intermediate component : something that escapes classification with either of two mutually exclusive and supposedly exhaustive categories but shares elements of both
God occupies merely an external relation, as a tertium quid, to mind and matter — James Martineau
where there are two systems of law and two orders of courts, there must obviously be some tertium quid to deal with conflicts of law and jurisdiction — Ernest Baker
2. : a third party of ambiguous status
once upon a time there was a man and his wife and a tertium quid — Rudyard Kipling