an unknown or indefinite thing related in some way to two known or definite things, but distinct from both: there is either right or wrong, with no tertium quid
[C18: from Late Latin, rendering Greek triton ti some third thing]
1724, from L., lit. "third something," loan-translation of Gk. triton ti (Plato), used in alchemy for "unidentified element present in a combination of two known ones."