A cocktail party was a gathering or social event usually centered around the serving of cocktails or other beverages.
Doctor Thomas Leighton and his wife Martha hosted a cocktail party at their home on Planet Q in 2266, in an attempt to gather information on Kodos the Executioner. (TOS: "The Conscience of the King")
The Wadi held a cocktail party in the third shap of their game Chula. (DS9: "Move Along Home")