Preternia is a term used in the Masters of the Universe brand that broadly refers to prehistoric times on the planet Eternia. Whereas Earth has a largely uninterrupted record of written history, modern Eternia usually has information only on events within living memory. Thus, while "prehistoric Earth" describes events that are at least thousands of years old, "Preternia" may refer to periods as recent as a hundred years prior to the lifetime of He-Man.
For storytelling purposes, many forms of Masters of the Universe continuity present characters and artifacts of ancient or mysterious origins, such as the Council of Elders or Castle Grayskull. These story elements are often stated or implied to predate some watershed event, such as the Great Wars, that divide modern Eternian civilization from the lost secrets and legends of prehistoric Eternia. "Ancients" and "elders" from these lost times are spoken of with reverence, suggesting that they possessed far greater power than is known in the present.
The term "Preternia" was originally developed as the setting for Mattel's canceled The Powers of Grayskull toyline, with He-Ro defending the ancients from the Snake Men in Preternia just as He-Man battles Skeletor in Eternia. The Masters of the Universe Classics toy line has expanded upon this premise, linking the backstories of Zodac, the Goddess, Hordak, and King Grayskull with this period.