Helmholtz Crater is an impact crater in the Argyre quadrangle on Mars at 45.8°S and 21.3°W and is 111.5 km in diameter. (wikipedia entry)
A crater is a circular depression likely created by an impact event. On Mars large craters are named after deceased scientists, writers and others who have contributed to the lore of Mars. Small creaters (less than 60km) are named after small towns and villages.