M2M (machine to machine)
Updated
M2M refers to communication between devices and systems without human involvement, for example a meter reporting to a utility server. IoT is the broader term that also covers platforms and applications; oneM2M is a global partnership that standardises a common service layer for both.
In practice
In connectivity offers, ‘M2M SIM’ and ‘IoT SIM’ mean the same thing: a SIM managed for devices, with a management interface, rather than a phone plan.