The term 'Geiger Counter' is a 6-letter solution to the crossword puzzle and refers to a radiation detector. It is named after physicist Hans Geiger, who invented the device in 1908. The Geiger Counter is an instrument used to detect and measure ionizing radiation, such as alpha particles, beta particles, and gamma rays. It works by detecting the ionization of gas molecules caused by the radiation, and then producing an audible or visual signal. The Geiger Counter is an important tool for measuring radiation levels in a variety of settings, including nuclear power plants, medical facilities, and research laboratories.