A sliding door was one which opened to the side, frequently automatically when people were sensed approaching. As Ianto Jones explained, they made use of wave bouncing detectors, which emitted radio waves and then looked for reflections in order to sense an approach. (TV: Out of Time)

On visiting his first space station, Marcipor was amazed to see a sliding door, and expressed wonder at it not opening outwards, as doors did in 1st century BC Rome. The Fifth Doctor was confused when it did not open automatically, as the door was stuck. (AUDIO: Interstitial)

On entering a Cardiff supermarket after being sent from 1953 to the 2000s, Diane Holmes was bewildered by the automatic sliding door. Ianto began to explain its method of operation, but Diane was distracted by bananas. (TV: Out of Time)

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