Used as a warm, affectionate way to refer to one's grandmother, showing fondness. My gran bakes the best cookies.
Pro Tip 2/3
Informal Usage
Preferable in casual or family conversations; 'grandmother' is more formal. Can I stay over at gran's place tonight?
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UK-Common
More frequently used in the UK; in other regions, alternatives like 'grandma' may be common. In the UK, kids often go to visit their gran after school.