maternal grandfather
Formal or written term for one's mother's father. In everyday speech, 母方の祖父 or simply おじいさん are more common.
My maternal grandfather often told me about his experiences during the war.
Compound of 外 (gai, 'outside, external') and 祖父 (sofu, 'grandfather'), literally 'external grandfather', reflecting the traditional patrilineal family system where the maternal line was considered the 'outside' family.