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.