Formal:
* Achan (അച്ഛൻ) - This is the most common and formal word for father, but it can also be used to address a grandfather respectfully.
Informal:
* Appan (അപ്പൻ) - This is a more informal and affectionate way to say father, and can also be used to refer to a grandfather.
* Paappu (പാപ്പു) - This is a very affectionate term for grandfather, often used by children.
* Thatha (താത്താ) - This is a more general term for grandfather, used in most contexts.
The best way to choose the right term depends on the situation and your relationship with the person.
For example, you might use "Achan" to address your own grandfather, but "Thatha" to talk about your friend's grandfather.