also: がいそん
noun
grandchild from a daughter married into another family
Refers specifically to the child of one's daughter who has married out of the family, as opposed to 内孫 (uchi-mago), the child of a son who remains in the family. This distinction is traditional and may not be observed in all modern families.
娘が嫁いだ先で生まれた子は外孫と呼ばれる。
A child born to a daughter who has married into another family is called a sotomago.
外孫は姓が違うので、家の跡継ぎにはならない。
Since a sotomago has a different family name, they do not become the heir of the household.
内孫 refers to a grandchild from a son who remains in the family, typically carrying on the family name. 外孫 is from a daughter who has married out, and thus belongs to another family.
Compound of 外 (soto, 'outside') and 孫 (mago, 'grandchild'), reflecting the traditional view that a daughter's child belongs to the husband's family.