Theaetetus (Moon)

Named after Theaetetus - Greek mathematician (c. 417-369 B.C.).

Theaetetus is a lunar impact crater that is located to the southeast of the crater Cassini near the eastern edge of Mare Imbrium. It lies just to the west of the Montes Caucasus range, which forms the eastern shore of the mare. To the southwest is the prominent crater Aristillus. The rim is distinctly polygonal in shape, with a slight rounding at the vertices. (wikipedia entry)

A crater is a circular depression likely created by an impact event. On the Moon they are named after deceased scientists, polar explorers, astronauts or cosmonauts.