Tacitus is a lunar impact crater located to the northwest of the crater Catharina, at the northern extension of the Rupes Altai ridge line. Directly west is the crater Almanon, and to the northeast is Cyrillus. To the southeast is a long chain of craters named the Catena Abulfeda. This chain runs to the northwest from the eastern edge of the Rupes Altai, continuing for over 200 kilometers. (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.