Traditionally the Dress Code Cocktail has been associated with dinner jackets and tuxedos but there are many more options than the most classic Black-Tie rig. The most conservative way would be to combine it with a white dress shirt with French cuffs, a black satin or grosgrain bow tie and black patent shoes. However, as society becomes less formal, Cocktail has today a much broader spectrum of options.
If you want a more casual but still elegant look there are a few key aspects to have in mind:
Evening wear and cocktail attire should always include dark colours so instead of a formal tux, you can choose a dark worsted suit in Navy or charcoal. Remember that darker colours have always been considered more formal than lighter ones and as much as one might like the beautiful look of casual cotton suit in olive green, they are in my opinion best worn during summer days. Same goes for warm shades of brown tweed or flannel which would be excellent during a daily activity in the fall. Worsted wool however has the subtle sheen and smartness that works exceptionally well for evenings during all four seasons.
Next thing to consider is the texture where the definition less is more comes to mind. For an evening outfit I would avoid fabrics with a significant texture and rugged feel. However, this doesn´t mean that the fabrics has to be shiny and options like high twist wools and fresco in darker shades can be wonderful with their matte finish.
Does cocktail necessarily mean a suit or tuxedo? In many cases I would say that this is the best way to go but a tailored blazer can most certainly be smart enough. It would still be best to stay within the darker colour spectra with Navy as the most obvious choice, but one exception of the rule could in my case be a jacket in camel hair. Worn with for example a cream or black lightweight rollneck and dark subtle trousers I think it can add a sophisticated touch to the cocktail attire. Take inspiration from the late designer Roy Halston, who made this specific outfit his signature look.
Cocktail indicates a few things: It´s festive, it´s often evening wear, but it’s not necessarily formal. As we touched upon in the previous point, some finer knitwear such as the turtleneck in a tonal look can be as smart as the more traditional shirt and tie. The latter is still an excellent choice but often considered business formal, and the day to day choice for someone working in a formal office. By swapping out the shirt and tie for a navy, black or burgundy turtleneck, you can transform the navy business suit into something chic and completely different for the evening cocktail.
Finally, like any dress code stated on an invitation, it is always wise to consult with the host if you are uncertain of what it means. Cocktail is not one of the traditional dress codes and could therefor imply everything from Black Tie to a tailored jacket but in general I would avoid strong colours and patterns to focus more on a dark and sophisticated monochromatic look.