pedometer; step counter
A device that counts the number of steps a person takes. The term is a registered trademark in Japan, but is widely used generically.
I wear a pedometer every day for my health.
Seeing the number on the pedometer, I decided to walk more.
A modern coinage from 万歩 “ten thousand steps” and 計 “meter.” Originally a product name of Yamasa Tokei Keiki, 万歩計 became widely used as a generic term for a pedometer.