The Greeks proposed the existence of five basic elements. Of these, four were the physical elements—fire, air, water, and earth—of which the entire world is composed. Alchemists eventually associated four triangular symbols to represent these elements.
The fifth element, which goes by a variety of names, is more rarefied than the four physical elements. Some call it Spirit. Others call it Aether or Quintessence (literally “the fifth element” in Latin).
Source: Catherine Beyer