Even in a restaurant or nightclub environment, 'business casual' indicates a relaxed but professional look -- something you might wear to the office on the day of a staff meeting.
For the ladies
For women, 'business casual' ensembles are made up of blouses, sweaters, or other fashion tops paired with slacks or knee-length skirts. Dresses that fall anywhere below the knee are also a great choice and can communicate casual elegance. Footwear should fall on the conservative side, with ballet flats or low heels taking center stage. Again,
you'll want to avoid anything too revealing, so a cardigan or blazer might be a good addition to the outfit to ensure the right level of modesty.
For the guys
A collared shirt, preferably a crisp button-down, is your best bet for a 'business casual' occasion. In warmer weather or in outdoor settings, a polo or tennis shirt could be acceptable if tucked in and paired with clean-cut dress pants and shoes. Jeans are too casual for this setting, so choose trousers or khakis and a leather belt. Footwear is similar to 'smart casual', so dress shoes like Oxfords and loafers are in, while sneakers are definitely out. Again, choose dark socks in a mid-calf length. 'Business casual' venues do not require ties, but if in doubt, go ahead and wear one -- you can always take it off later. Whether or not to wear a jacket, like a sport coat or blazer, is totally up to you.