What is Aristotle's fifth substance?

Answer 1

The ancient Greeks and Romans believed that all matter was made up of varying amounts of four elements - Earth, Air, Fire and Water... these were seen as essential... the quintessence, the fifth essential was also in all things, btu in particular was in the heavenly bodies. Quin~ is the prefix for 5.

Answer 2

Aristotle's fifth substance is the quintessence, also known as aether. This concept was believed to be the substance that made up the heavenly bodies and was considered to be eternal and unchanging. It was thought to be superior to the four earthly elements.