A modem is a device that carries data over communication lines by converting digital information into a signal format that can travel across the medium, then converting it back at the other end. The name comes from modulator-demodulator, which describes that signal conversion process.
In cyber security, modems matter because they create the link between a local device and an outside network. That link becomes part of the attack surface: exposed management interfaces, weak default credentials, misconfigured bridging, or unsupported firmware can all be abused. In retro systems, a modem add-on can be especially sensitive because older designs often assumed limited connectivity and trusted endpoints. Defenders treat modem paths like any other network boundary by restricting access, monitoring traffic, and verifying what hardware or relay service is really handling the connection.