Formal
It's both formal and informal. It depends on how you say it. If you say "Adiós, señora." it's formal. If you say "Adiós, amigo." it's informal.
Adios Senor - 1941 was released on: USA: 31 December 1941
= 'How are you, sir?' (Mixture of informal 'estas' and more formal 'senor') You'd have expected the more (consistently) formal: 'Como esta usted, Senor?' In both cases, 'esta(s)' should have an accent on the 'a'. If it doesn't, it could also mean: Like these, senor? ('Like' in sense of 'similar to') I eat these, senor.
it is an informal group
formal is the cinema and informal is a gay man
"Adios" is a Spanish term used to say goodbye or farewell. It is a formal and respectful way to part ways with someone.
informal
"Hasta luego" is neither formal nor informal, it straddles the middle between them and can be used in most situations. If you want a truly formal way of saying goodbye, "Adios" is probably the most formal. "Chao" correspondingly, should only be used between close friends or family members who are your age or younger.
formal and informal rhythm
what is formal and informal communication
formal