A noun meaning grandfather — the father of one's parent; commonly used in family contexts.
私の祖父は戦争の話をよくしてくれた。
My grandfather often told me stories about the war.
毎週日曜日は祖父の家に行く。
I go to my grandfather's house every Sunday.
祖父の写真をアルバムに入れた。
I put my grandfather's pictures into the album.
Sometimes used to mean an old man or elderly person, depending on context (somewhat colloquial or older usage).
公園で座っている祖父たちが将棋を指している。
Old men sitting in the park are playing shogi.
あそこにいる祖父が店の常連だ。
That old man over there is a regular at the shop.
In traditional performing arts (e.g., kyogen), it can refer to a mask used for the role of an old man — a specialized usage.
能楽の公演では祖父の面が使われた。
In the Noh performance, a grandfather (old man) mask was used.