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tertium quid (n.)

something indeterminate between two other things, 1724, Latin, literally "third something," from tertius "third, a third," from the root of tres "three" (see three). A loan-translation of Greek triton ti (Plato), used in alchemy for "unidentified element present in a combination of two known ones." The Latin word also figures in phrases tertium non datur "no third possibility exists," and tertius gaudens "a third party that benefits from conflict between the other two."

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Definitions of tertium quid from WordNet

tertium quid (n.)
some third thing similar to two opposites but distinct from both;
From wordnet.princeton.edu