What is "tertium quid"

Tertium quid \Ter"ti*um quid\ [L.] A third somewhat; something mediating, or regarded as being, between two diverse or incompatible substances, natures, or positions.

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."

n. something intermediate between two classifications

n. some third thing similar to two opposites but distinct from both

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.