A child born to keep another child from being an only child or to provide transplantable parts for an older child, especially one who would not have been conceived otherwise.
In Jodi Picoult's novel My Sister's Keeper, Anna Fitzgerald, the daughter conceived to provide bone marrow and a kidney transplant to her terminally-ill older sister Kate, is the add-on child of the family.
by JavaJaneOhio December 8, 2010