Brewer's: Tertium Quid
A third party which shall be nameless. The expression originated with Pythagoras, who, defining bipeds, said -
?Sunt bipes homo, et avis, et tertium quid.?
?A man is a biped, so is a bird, and a third thing (which shall be nameless).?
Iamblichus says this third thing was Pythagoras himself. (Vita Pyth., cxxvii.) In chemistry, when two substances chemically unite, the new substance is called a tertium quid, as a neutral salt produced by the mixture of an acid and alkali.