IMU (inertial measurement unit)

The IMU tells the robot how its body is tilted and how fast it is turning, like the inner ear of a person. A walking robot reads it hundreds of times per second to stay upright, and a wheeled robot uses it to estimate how far it has moved between map updates.

IMUs drift over time, so they are combined with other sensors. You rarely choose an IMU yourself, but it is a sign of quality when a vendor names the unit and states the update rate. Its absence on a legged robot would be a red flag.