My grandfather was a farmer.
Oxford Essential Dictionary
grandfather
(also informal) grandpa ) noun
the father of your mother or father
Longman Dictionary of Contemporary English
grandfather
grand‧fa‧ther S3 /ˈɡrændˌfɑːðə $ -ər/ BrE AmE noun [countable]
the father of your father or mother
REGISTER
People 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.
Oxford Advanced Learner's Dictionary
grand·father [grandfather grandfathers] [ˈɡrænfɑːðə(r)] 
[ˈɡrænfɑːðər] 
noun
the father of your father or mother
Cambridge Advanced Learner's Dictionary
grandfather /ˈgrænd.fɑː.ðəʳ/ US /-ðɚ/
noun [C] (INFORMAL grandpa or grandad)
the father of a person's mother or father
Collins Advanced Learner’s English Dictionary
grandfather
/grændfɑ:ðə(r)/
(grandfathers)
Your grandfather is the father of your father or mother.
His grandfather was a professor.
Merriam-Webster's Advanced Learner's Dictionary
grandfather
grand·fa·ther /ˈgrændˌfɑːðɚ/ noun, pl -thers [count] : the father of your father or mother