The Free On-line Dictionary of Computing (30 December 2018):
A type of computer peripheral connector so named because one side is shorter (with one less pin) than the other giving a (squarish) "D" shape. The connectors have two rows of pins (or holes). Common types are 25-way (13+12 pins) and 9-way (5+4 pins). They are often used for serial lines, especially EIA-232. (1995-01-05) 2. D-type flip-flop.