In cybersecurity, 'serial' refers to data transmission using a serial communication protocol, where data is sent one bit at a time over a single channel or wire. Serial connections are commonly used in older hardware interfaces, such as RS-232 or RS-485, to connect devices like modems, sensors, or industrial equipment. These connections can be vulnerable to security risks if not properly managed, as they often lack encryption and authentication. Modern cybersecurity practices may involve securing serial communications or using to-IP converters to integrate legacy serial devices into secure, IP-based networks.