Abbreviation for "duplex drive", a modification to tanks to make them amphibious, developed by the British military inventor Major General Percy Hobart during the Second World War.
The eponymous drive was basically just a pair of propellers linked up with the transmission which could drive the vehicle through the water at a speed of around four knots. The more visually striking part of the system, however, was the pneumatically-erected canvas flotation skirts. These rose from a frame mounted around the hull of the tank to rise a couple of metres above the top of the turret; the tank itself floated below the waterline, so that all was visible was what looked like a shallow canvas boat. The tank was steered by the driver altering the relative speed of the two propellers (not dissimilar to the way you steer with tracks); the tank commander could just see out to navigate by standing on top of the turret. The crew were issued with primitive breathing apparatus in order to try and escape if the vehicle was sunk.
The system was initially tested on Valentine and Tetrarch tanks, but eventually saw action as a modification to the ubiquitous M4 Sherman. In the preparations for the D-Day landings DD tanks were allocated to the British army's 79th Armoured Division, a specialist amphibious assault and engineering formation which also operated many other vehicles of Hobart's invention, to some units of the Canadian army, and (alone) of Hobart's "funnies", to the US army. On D-Day itself several regiments of DD tanks were launched from 4 km off shore and fought with considerable success on four out of the five landing beaches, but most of the tanks in the US 741st Armored Battalion allocated to Omaha beach were swamped after having navigational problems which led to their getting beam-on to the fairly rough seas.
Although D-day used up the element of surprise for the design, DD tanks were subsequently used in various river crossings, including the attack across the Waal to capture Nijmegen bridge during the Arnhem operation.