A Radio Access Technology or (RAT) is the underlying physical connection method for a radio based communication network. Many modern phones support several RATs in one device such as Bluetooth, Wi-Fi, and 3G, 4G or LTE.
The term RAT was traditionally used in mobile communication network interoperability, for example, in Virtej et al.,[1] or in the example provided on the Wiki page for Radio access network.
More recently, the term RAT is used in discussions of heterogeneous wireless networks. The term is used when a user device selects between the type of RAT being used to connect to the Internet. This is often performed similar to access point selection in IEEE 802.11 (Wi-Fi) based networks.[2]

Source: Radio access technology – Wikipedia