Tertium quid

Tertium quid refers to an unidentified third element that is in combination with two known ones. The phrase is associated with alchemy. It is Latin for "third thing", a translation of the Greek tríton ti (τρίτον τι). The Greek phrase was used by Plato (360 BC), and by Irenæus (c. AD 196). The earliest Latin example is by Tertullian (c. 220), who used the phrase to describe a mixed substance with composite properties such as electrum, a somewhat different sense than the modern meaning.

In the Christological debates of the fourth century, it was used to refer to the followers of Apollinaris who spoke of Christ as something neither human nor divine, but a mixture of the two, and therefore a "third thing".

In American political history, the Tertium Quids, or Quids, were moderate members of Jefferson's Democratic-Republican Party. The word implies that their political position was apt to embrace true republicanism and the comparable conservatism of the Federalist Party, particularly on foreign policy. The Quids arose in 1804 during Thomas Jefferson's first term in office. They were led by Virginia's John Randolph of Roanoke. They stood by the party's original stance for strict construction of the Constitution and opposed Jefferson's pragmatic approach to governing.

In sociology, it describes a category of degraded moral consideration.

In Souls of Black Folk, W.E.B. Du Bois used the term "tertium quid" to refer to the identity of African Americans in a racist society, where non-white people are viewed as a devalued category between man and animal.

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