formal — being in accordance with the usual requirements, customs, etc.; conventional: to pay one's formal respects.
Three-syllable rhymes
abnormal — Someone or something that is abnormal is unusual, especially in a way that is worrying.
informal — without formality or ceremony; casual: an informal visit.
Four-or-more syllable rhymes
paranormal — of or relating to the claimed occurrence of an event or perception without scientific explanation, as psychokinesis, extrasensory perception, or other purportedly supernatural phenomena.
