Tertium quid

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Tertium quid (Latin for a "third thing") was a term used in the Christological debates of the fourth century 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".

This term is also used in the important Supreme Court case Wal-Mart Stores, Inc. v. Samara Brothers, Inc. 529 U.S. 205 (2000). In this Lanham Act case, the court, when discussing product packaging vs. product design, referred to the type of trade dress in its earlier Two Pesos decision as "some tertium quid" that may be a mutation of product packaging and product design - a "third thing." This is obviously a non-religious usage.