Not in the regular, usual, or established form; not according to official, conventional, prescribed, or customary forms or rules; irregular; hence, without ceremony; as, an informal writting, proceeding, or visit. Deranged in mind; out of one's senses.
Colloquial casual unofficial casual
Formal
Not formal or ceremonious. Not in accord with the usual regulations. Suited for everyday use. (of language) Reflecting everyday, non-ceremonious usage. Not organized; not structured or planned.