A handheld or worn device that counts the number of steps you take is called a pedometer. It is also commonly known as a step counter, and modern versions are built into smartphones, watches and activity trackers.
Traditional pedometers used a mechanical pendulum or switch that moved whenever the wearer took a step. Contemporary devices generally use accelerometers and software to recognize patterns of movement, making them smaller and more adaptable to different carrying positions.
A pedometer can estimate distance by multiplying the number of steps by a user’s stride length, but that estimate is not identical for everyone. An odometer measures distance travelled by a vehicle or machine, while a barometer measures air pressure and an ergometer measures work or exercise output.