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According to reference.com, “cocktail casual” is not formally defined: “Traditional cocktail attire usually consists of knee length dresses for women, dark suits with a tie for men and fancy shoes for both. Cocktail casual, however, [...] has not yet been concretely defined. It means different things to different people, so partygoers who know the host well should feel free to ask for more specific instructions.”

Knee-length dresses (i.e. “short dresses”) seem to reasonably fall under the definition of “cocktail casual”. For example: https://www.clbxg.com/Cocktail-Casual/