Short for communications port , the logical address assigned by MS-DOS (versions 3.3 and later) and Microsoft Windows (including Windows 9x and Windows NT) to each of the four serial ports on an IBM Personal Computer or a PC compatible. COM ports also have come to be known as the actual serial ports on a PC’s CPU where peripherals, such as printers, scanners, and external modems, are plugged in. See the illustration. See also COM (definition 1), input/output port, serial port.