Formal:
* पितृमहः (pitṛmahaḥ): This is the most common and formal term for grandfather. It literally means "father of the father".
* आजीमहः (ājīmahaḥ): This is another formal term meaning "great-grandfather".
Informal:
* दादा (dādā): This is a more informal and common term used for grandfather, particularly in North India.
* नाना (nānā): This is another informal term used for grandfather, particularly in South India.
Other:
* पितृपुरुषः (pitṛpuruṣaḥ): This literally means "father-man" and can be used for grandfather in a more general sense.
* महतः (mahaḥ): This is a term of respect that can be used to address an older person, including a grandfather.
The best term to use will depend on the context and the relationship you have with the person.