From King Dictionary of Contemporary English
Related topics: Family
grandfathergrand‧fa‧ther /ˈɡrændˌfɑːðə $ -ər/ ●●● S2 W3 noun [countable] SSFFAMILYthe father of your father or motherRegisterPeople usually say grandad (especially in British English) or grandpa when talking to or about their own grandfather:I used to stay at my grandpa’s house.
Examples from the Corpus
grandfatherAnd grandfather caught a bus up to Ipswich market where my father was, and they drove home.She was excited to see her grandfather and uncle, running to hold up her face for a kiss.Through his stories about his grandfather, he had long ago learned to survive by his wits.Leslie and his grandfather collected Sam and carried him to Lovettsville.Another man kills his grandfather for five dollars.His grandfather enjoyed a gamble - the trick all along was to keep on increasing the size of the farm.All the men he knows are fathers or grandfathers and it is in that capacity that he knows them.