Formal:
* Дедушка (dedushka): This is the most common and respectful way to say "grandfather" in Russian.
Informal:
* Деда (deda): This is a shortened, informal version of "dedushka," used with close family members or friends.
* Батя (batya): This word literally means "father" but can be used as a familiar term for "grampa," especially in some regions of Russia.
Other options:
* Старик (starik): This literally means "old man" and can be used as a term of endearment for a grandfather, but it can also be disrespectful depending on the context.
* Дядя (dyadya): This word means "uncle," but can also be used to refer to a grandfather, especially if he is not a direct blood relative.
The best way to choose the right term depends on your relationship with the person you are talking about and the level of formality you want to convey.