Definition of "formal"

formal¹

/ˈfɔɹməl/

adjective
comparative more formal, superlative most formal
1. Being in accord with established forms.
2. Official.
3. Relating to the form or structure of something.
  Quotations
  THE THREE DOMINANT FORMS IN METALSMITHING […] At present, there are but three basic volumetric forms dominating the work of metalsmiths, the spherical (usually in its most practical form, the domical), the cylindrical, and the cubical. […] The possibilities for further variations on them are all but exhausted, there being little chance to express new and unusual ideas within the framework of such limited choices. As a result, much of twentieth-century metalsmithing has relied on surface enrichment rather than formal development for its originality.
  1978, Heikki Seppä, Form Emphasis for Metalsmiths, [Kent, Oh.]: Kent State University Press, page 1
4. Relating to formation.
5. Ceremonial or traditional.
6. Proper, according to strict etiquette; not casual.
7. Organized; well-structured and planned.
8. (mathematics) Relating to mere manipulation and construction of strings of symbols, without regard to their meaning.
noun
countable and uncountable, plural formals
1. (clothing) An evening gown.
  Quotations
  Well, I open up the door / And much to my surprise / The girls were wearin' formals / And the boys were wearin' ties
  1965, Shadow Morton (lyrics and music), “Sophisticated Boom Boom”, performed by The Shangri-Las
2. An event with a formal dress code.
3. (programming) A formal parameter.
4. (Oxbridge slang) Ellipsis of formal hall..

formal²

noun
countable and uncountable, plural formals
1. (uncountable) Formalin.
2. An acetal formed from formaldehyde.