Earth, air, fire and water (sometimes accompanied by a fifth element, aether).
These are the substances that Western people from the ancient Greeks through the Middle Ages believed the physical world to be made. Everything was supposed to be some mixture of earth, air, fire, and water. Many Greek philosophers (with exceptions, like Democritus, who coined the term "atom") believed in these elements, but they are named after Aristotle because he was the most famous.
No one seriously believes in them any more, but people believed in them for so long that they still show up in Western culture in astrology and in works of fiction set in medieval times, like the popular Dungeons and Dragons games.
Aether is supposed to be quintessence, the uncorruptable material that makes up the stars. It's loosely comparable to plasma. From this word, we get the modern word "ether," which means both the chemical ether and also "sky"/"air"/"nothingness." (What people thought air was before realizing that it had matter.) Any pre-modern concept of the elements that is based on their observable traits (such as phases of matter), is called classical elements. Some cultures have five (e.g. Chinese culture) or seven classical elements instead of four. Some have just one.(Please retain the reference in reprint: http://www.letpub.com/index.php?page=author_education_aristotelian_elements)