A grandchild; the child of one's son or daughter.
おじいさんは孫と一緒に公園へ行った。
Grandpa went to the park with his grandchild.
娘に孫が生まれて、家族みんなで喜んだ。
A grandchild was born to my daughter, and the whole family was delighted.
週末は孫の世話をして過ごすことが多い。
I often spend weekends taking care of my grandchild.