Tiselius is a lunar impact crater that lies just to the east of Valier,. The craters Tiselius and Valier are separated by only a few kilometers. Less than one crater diameter to the east is the smaller, elongated Stein, and to the north is the small, eroded Šafařík. This is a roughly circular crater with a well-defined edge that has not been significantly degraded by impact erosion. (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.