The business suit is a traditional Western-style business attire. For men, this usually consists of a jacket, worn over a collared shirt, with a matching pair of trousers; a more formal setup also includes a waistcoat vest. For women, this would usually be a jacket, worn over a blouse, with a matching skirt (commonly a pencil skirt) or trousers. This is considered a relatively conservative mode of dress, and is the sort of attire that a typical Japanese office worker would use on a typical working day.
Not to be confused with a blazer or sport coat, both of which are similar in cutting, but more casual in overall style.