the father of one's son- or daughter-in-law, as common grandfather of their children; the relationship between men who have grandchildren in common; maternal grandfather vis-à-vis paternal grandfather.

plural of co-grandfather

A grandfather of one's spouse.

plural of maternal grandfather

plural of great-grandfather

plural of grandfather clock

plural of grandfather paradox

The paradox of time travel in which inconsistencies emerge through changing the past.

plural of great grandfather (alternative spelling of great-grandfathers).

plural of grandfather-in-law