Terʹtium Quid.

A third party which shall be nameless. The expression originated with Pythagoʹras, who, defining bipeds, said—

“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.)

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.

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Entry taken from Dictionary of Phrase and Fable, edited by the Rev. E. Cobham Brewer, LL.D. and revised in 1895.