Formal: Pre-established acts or words

So it is something which is used or done in accordance with rules or ceremonies.

Formal events have rules. 'Formal clothes' or 'formal attire' is a way of dressing to meet the rules for an event, such as a party or wedding. 'Formal attire' can be a suit, a tuxedo or a dress. People from all over the world have their own ideas about 'formal attire'. Most of the time formal attire clothes are the most beautiful or the best clothes a person owns. The opposite of "formal" is informal. Informal clothes are comfortable clothes for general use, e.g. T shirt and jeans. An informal ceremony might be a ceremony which is very relaxed instead of following an exact plan. An informal way to greet a friend might be: "Hi, John!"

Examples:

• Formal can be the way a person speaks or acts. To act in a formal way a person will follow the social rules of their culture or country. For example, they might greet each other by bowing, and they might address one another by their family name and their title (e.g. "Professor Smith").

• A formal agreement is one which is explicit (written down) and reached by a formal discussion or negotiation. That means the negotiation is also done by set rules or procedures.

• In mathematics, formal logic is a way of thinking about mathematics questions using strict rules.