Longman Dictionary of Contemporary English - formal

~1 adj 1 formal behaviour is very polite, and is used with people you do not know well, or in official situations or at important social occasions  (Our boss is very formal, she doesn't call anyone by their first name.) 2 formal language is used in speeches, in serious or official writing, or at official meetings or important social occasions  (You shouldn't use `Yours faithfully' in a letter to a friend - it sounds too formal.) 3 a formal occasion is serious and important, and people who go to it wear good clothes and behave according to strict social rules  (I only wear this suit for formal dinners.) 4 a formal decision or action is made or done officially or publicly  (On July 19th a formal declaration of war was made in Berlin.) 5 formal education/training/qualification education etc in a subject or skill gained in a school, college etc rather than practical experience of it  (Many of the health workers had no formal medical training.)  (- see also formally) ~2 n AmE 1 a formal social event such as a dance, at which you have to wear formal clothes 2 an expensive and usually long dress that women wear at a formal occasion