A cocktail dress was an elegant dress suitable for smart social occasions, falling to just below or just above the knee.
Ruby Sunday wore a black and white cocktail dress during a visit to London during 1963. (TV: The Devil's Chord)
When exiting the Celestial Omnibus when Iris Wildthyme and Panda arrived at the cathedral, Iris wore a cocktail dress and wellies. (PROSE: Iris Wildthyme and the Unholy Ghost)