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
It was a formal even, which meant everyone would have to dress up in their best clothes and e on their best behavior.
The school held a formal dance where all students were required to be dressed in slacks and button ups shirts or suits for the boys and dresses/gowns for the girls.
His attire was much less formal than all of the other party goers, and this made him feel like a slob.