tertium quid

noun

ter·​ti·​um quid ˌtər-shē-əm-ˈkwid 

ˌtər-tē-

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

Word History

Etymology

Late Latin, literally, third something; from its failing to fit into a dichotomy

First Known Use

circa 1724, in the meaning defined at sense 1