what is an inertial measurement unit(IMU)?

Where can we find an IMU sensor?
Actually, almost everywhere! Let’s start with our smartphones. Most smartphone devices are equipped with an IMU sensor inside, a MEMS (Micro-electromechanical Systems) technology. It is also placed in many Tablet devices. The IMU is very common in the automotive and aerospace industries, as it allows a better determination of the vehicle position and orientation.

General Description
The Inertial Measurement Unit is one of the common sensors in the navigation field (but not only). It contains an accelerometer and gyroscope (sometimes also a magnetometer and rarely also a barometer). The first is responsible for acceleration measurements and the second is responsible for angular velocity measurements. Each one of the measures is represented in a three-axis coordinate system, so generally speaking, they both together yield a 6-dimension measurement time series stream.