A Geiger counter was a device used to measure radiation.
When he was transported from 2020 to 2270, Shaun Christopher saw Spock holding a tricorder and believed it looked like a Geiger counter. (TOS novel: The Rings of Time)
In 2366, Geordi La Forge compared the merger of his VISOR and a tricorder to a Geiger counter. (TNG episode: "The Enemy")