In conformance with generally accepted standards; the opposite of casual.
Generally accepted formal standards usually denote professionalism, whereas the absence or lack of standards would be seen as casual. For example, a business jacket, tie, button-up shirt, slacks, dark-colored socks, business shoes, and a neat and clean appearance is considered formal attire for men. Similarly, a formal letter would have many different characteristics than a casual email, such as a standard header, salutation, closing, and formatting.
Use formal in a sentence