dowager (n.)

1520s, "title given to a widow of rank to distinguish her from the wife of her husband's heir bearing the same name," from Middle French douagere "widow with a dower" literally "pertaining to a dower," from douage "dower," from douer "endow," from Latin dotare, from dos (genitive dotis) "marriage portion, dowry," from PIE *do-ti, from root *do- "to give."

"App. first used of Mary Tudor, widow of Louis XII; then of Catherine of Arragon, styled 'Princess Dowager'" [OED]. In law, "a widow possessed of a jointure."