There is no confirmed unified definition each developer, vendor or CSP have own definition of it.
Machine to Machine by Telit
Machine to machine communication (m2m) is the term that identifies the space enveloping the collection of devices, services and the value chain required to allow the interconnection of electronic apparatus, typically but not always wirelessly. Another characteristic of m2m communication is that this interconnection enables primarily automated communication between distant, remote machines and one or more layers of central management applications. It provides for real-time monitoring and control without the need for human intervention.