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 something" (literally, "third what"), a translation of the Greek triton ti (τρίτον τί). 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.