Formal means serious and official — the style of weddings, courts and job interviews. Formal dress, a formal complaint, formal language.
In clothing it means the smartest register: formal wear, black tie. In language it means the careful written style with no contractions and no slang, which is what learners are usually taught first. A formal complaint or formal warning is one that enters an official record, which is what makes it formal rather than a conversation. Note formality, meaning both the quality and a procedure done only because it is required.
1adjectiveformalSerious and official in style.
“The invitation said formal and half the room ignored it.”
2adjectiveenteringEntering an official record.
“She made a formal complaint and it was upheld.”
formal is a quality of officialness.
Latin formalis means to do with the form rather than the substance. That is why a formality is something done for the shape of the thing, and why formal dress is about the outward form of an occasion.
Latin
fōrmālis
Old French
formel
Middle English
formel
English, today
formal
Built from
From Middle English formel, borrowed from Old French formel, from Latin fōrmālis, from fōrma (“form”); equivalent to form + al.
Etymology from Wiktionary, CC BY-SA