The 5-pin DIN keyboard connector used with the first IBM PC (1981) and second-generation IBM AT (1984). It was subsequently replaced with the smaller 6-pin Mini-DIN connector on the IBM PS/2 in 1987. The 6-pin Mini-DIN was first used on laptops. See PC keyboard
AT and PS/2 Connectors
AT-to-PS/2 adapters let newer keyboards work with old computers and vice versa. Both of these plugs and sockets were replaced by USB.