Aetiological (sometimes spelled etiological) myths explain the reason why something is the way it is today. The word aetiological is from the Greek word aetion (αἴτιον), meaning “reason” or “explanation”. Please note that the reasons given in an aetiological myth are NOT the real (or scientific) reasons. They are explanations that have meaning for us as human beings. There are three subtypes of aetiological myths: natural, etymological, and religious.
A natural aetiological myth explains an aspect of nature. For example, you could explain lightning and thunder by saying that Zeus is angry.
An etymological aetiological myth explains the origin of a word. (Etymology is the study of word origins.) For example, you could explain the name of the goddess Aphrodite by saying that she was born in sea foam, since aphros is the Greek word for sea foam.
A religious aetiological myth explains the origin of a religious ritual. For example, you could explain the Greek religious ritual of the Eleusinian Mysteries by saying that they originated when the Greek goddess Demeter came down to the city of Eleusis and taught the people how to worship her.
All three of these explanations are not true: Zeus’ anger is not the correct explanation for lightning and thunder, Aphrodite’s name was not actually derived from the Greek word aphros, and Demeter did not establish her own religious rituals in the town of Eleusis. Rather, all of these explanations had meaning for the ancient Greeks, who told them in order to help themselves understand their world.