A formal occasion is an event in which a person’s behavior reflects serious or important reasons for being there. Men wear suits and women wear dresses for formal occasions. When something is formal, activities might be rehearsed or so well-practiced that people know well what their roles and responsibilities are.
A wedding is usually a formal occasion.
People wear formal attire when attending a funeral. (attire = clothing)
Office attire is formal, but it depends on a company’s policies regarding dress.
We received a formal letter inviting us to attend a ceremony.
Everyone should learn how to write a formal letter.
A professor uses a formal manner of speech when giving a lecture.
Students in elementary school and high school address their teachers in a formal manner, using Mr. , Miss, Mrs. or Ms. and then the teacher’s last name.
My name is Mr. Lawrence, but you don’t have to be so formal. Just call me Paul.
Americans generally are very informal. (The opposite of "formal" is "informal.")
The word "formally" is an adverb:
Ron and Sara were formally introduced to each other.
We weren’t formally introduced, but I know who you are.
A man was formally charged by the county prosecutor for violating laws related to the construction of new buildings.
Office workers these days dress less formally than they did twenty years ago.
A man who wears a tuxedo is formally dressed for an important occasion.