Formal:
* Старший брат (starshiy brat) - This is the most formal and general term for "older brother".
Informal:
* Брат (brat) - This is the most common way to say "brother", and it can be used for both older and younger brothers. You can add "старший" (starshiy) before it to make it clear you're referring to the older one.
* Братан (bratan) - This is a very informal and friendly term for "brother", usually used for close friends. It can be used for both older and younger brothers, but it's more common to use it for older ones.
Example:
* У меня есть старший брат. (U menya est starshiy brat.) - I have an older brother.
* Мой брат — врач. (Moy brat — vrach.) - My brother is a doctor.
* Привет, братан! (Privet, bratan!) - Hey, brother! (informal)
The choice of term depends on the relationship between the speaker and the listener, as well as the context of the conversation.