Apple coprocessorA motion processor in the iPhone, starting with the M7 chip in the iPhone 5S. Designed for tracking the user's movement, the Apple motion coprocessor is a low-power unit that reads the output of the phone's accelerometer, gyroscope and compass and makes that data available to applications via Apple's CoreMotion programming interface (API). For example, it enables fitness apps and navigation apps to track movement for long periods of time without seriously draining the battery. See iPhone 5S.
For the iPhone 6, Apple's M8 coprocessor can determine if the user is running or biking. Its barometer can measure elevation, and it can track travel distance.
Starting with the iPhone 6s and adding support for Siri commands, Apple's M9 coprocessor was embedded in Apple's A9 system-on-chip (SoC). The subsequent M10 is part of Apple's A10 SoC for the iPhone 7. See iPhone 7, iPhone 6 and Apple A series.