A cocktail dress is the appropriate attire for a cocktail party, but you have other options. A top with special details or embellishment around the bust and a simple skirt or tailored pants, plus heels or fancy flats can be worn. Fabrics like jersey and denim should be avoided. If the cocktail party is work related (organized by your firm), avoid being too corporate-ish. A pantsuit or a fitted cashmere top with a knee-length satin skirt, heels, earrings, and stacked bangles are perfectly fine. Wear a silky top if the pantsuit is your outfit of choice.
Cocktail parties are different from cities to cities. A cocktail party in New York city -where people like to look chic at social events- might be different from a cocktail party in a more relaxed city. Figure out what people usually wear at cocktail parties in your area so you don't look over-dressed or otherwise. The main reason people have cocktail parties is to relax and have fun. So dress comfortably and drink on!