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.
Formal
"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
Formal
Usted is the formal way of saying 'you'.
Formal
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
formal and informal rhythm
what is formal and informal communication
formal
"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.
informal
Formal is polite, informal is rude and obnoxious.
There is no formal slang. Slang is informal language.