E. Cobham Brewer 1810–1897. Dictionary of Phrase and Fable. 1898.
Ter’tium Quid.
A third party which shall be nameless. The expression originated with Pythago’ras, who, defining bipeds, said—
1
“Sunt bipes homo, et avis, et tertium quid.
“A man is a biped, go is a bird, and a third thing (which shall be nameless).”
Iamblichus says this third thing was Pythagoras himself. (Vita Pyth., cxxvii.)
2
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.
