A motion processor in the iPhone, starting with the M7 chip in the iPhone 5S in 2013. Designed for tracking user movement, the Apple coprocessor is a low-power unit that sends the output of the phone's accelerometer, gyroscope and compass to applications via Apple's CoreMotion programming interface (API). For example, the M coprocessors enable fitness and navigation apps to track movement for long periods of time.
The M7 and M8 were separate chips, but M9 through M12 were built into the A series, which are the primary iPhone chip. After M12, Apple stopped advertising the M coprocessors in order not to conflict with the Apple-designed M chips. See
Apple A series and
Apple M series.