Grandfather — a polite or affectionate term for one's own or someone else's grandfather.
おじいさんの家に夏休みに遊びに行った。
I went to visit my grandfather's house during summer vacation.
お祖父さんは昔から魚釣りが得意だ。
Grandpa has been good at fishing since long ago.
駅でおじいさんが道に迷っていたので、案内した。
An old man was lost at the station, so I helped guide him.
An old man; a male senior citizen — used to refer to elderly men (not necessarily one's grandfather).
公園でおじいさんたちが将棋を指していた。
Old men in the park were playing shogi.
バスで席を譲ってほしいと、若者におじいさんが頼んだ。
An old man asked a young person on the bus to give up a seat.