Tertium quid
![Electrum, a rare alloy of gold and silver, was used by Tertullian as an example of a tertium quid, by which he meant a mixture with composite properties.](/Images/godic/202502/15/Gold-1217003121.jpg")
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.