tertium quid
Concise Oxford English Dictionary © 2008 Oxford University Press:
tertium quid /ˌtəːʃɪəm ˈkwɪd, ˌtəːtɪəm/
▶noun an indefinite and undefined third thing related to two definite or known things.
– origin C18: from late L., translation of Gk triton ti ‘some third thing’.
'tertium quid' also found in these Oxford entries:

