A modem that plugs into the USB port of a computer. The term states only that the connection to the computer is via USB but does not imply which network service is supported. It often refers to a 3G or 4G cellular modem, but it may also refer to an analog dial-up modem.
It generally does not refer to a cable or DSL modem, because the vast majority of those devices connect to the computer or the network via wired Ethernet. See cellular modem
, analog modem
Although both of these devices are advertised as USB modems, the Sewell unit (top) is a dial-up modem for an analog phone line, while the Verizon device adds 3G data service to a computer or tablet. (Images courtesy of Sewell Development Corporation and Verizon Wireless.)