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Serial port
A socket on a computer used to connect a modem, data acquisition terminal or other device via a serial interface (one data bit following the other). Serial ports provide very slow speeds and have been superseded by USB and other faster interfaces for peripheral connections to desktop computers. Although still widely used in data acquisition, the serial port is no longer found on new computers. Earlier PCs used the port for the mouse, and earlier Macintoshes used it to attach a printer.
DB (D-Sub) Connectors and COM Ports
If present, the serial port on the back of a PC is a male 9-pin connector (DE-9 D-sub connector). Earlier PCs may have had two 9-pin connectors or one 9-pin and one 25-pin (DB-25). On a PC, serial ports are called "COM ports," identified as COM1, COM2, etc. See COM1 and D-sub connectors.

Serial, Parallel and Game
In earlier PCs, one or two serial ports, one parallel port and one game port were included on the motherboard. On the first PCs, these ports were contained on a stand-alone expansion card plugged into the ISA bus. Contrast with parallel port. See serial interface and RS-232.

Faster, But Still Serial
The USB and FireWire (IEEE 1394) interfaces were added to PCs in 1998, offering a quantum jump in transfer rate, plus the ability to daisy chain large numbers of devices on the same bus. Like the legacy serial port, USB and FireWire are also serial interfaces. See USB and FireWire.
In Transition

As USB ports (left) began to proliferate, the serial and parallel ports (right) were included for a while, but eventually gave way entirely to USB.