A dowager is a widow holding a title or property that was her late husband's. The root word is also found in the word "endow."
A living female who is an ancestor of the current holder of a title is also termed a dowager.
An example: the Dowager Empress Cixi, a widow of an emperor, ruled China in the place of first her son and then her nephew, both titled Emperor.
Among the British peerage, a dowager continues to use the female form of her late husband's title so long as the present male title-holder does not have a wife. When the present male title-holder marries, his wife assumes the female form of his title and the title used by the dowager is the female title prepended with Dowager ("Dowager Countess of ...") or by using her first name before the title ("Jane, Countess of ...").
The title "Dowager Princess of Wales" or "Princess Dowager of Wales" was given to Catherine of Aragon when Henry VIII arranged to annul their marriage. This title refers to Catherine's previous marriage to Henry's older brother, Arthur, who was still Prince of Wales at his death, widowing Catherine. At the time of the marriage of Catherine and Henry, it was alleged that Arthur and Catherine had not consummated their marriage due to their youth, freeing Henry and Catherine to avoid the church prohibition on a marriage to one's brother's widow. At the time Henry wanted to obtain an annulment of the marriage, he alleged that Arthur and Catherine's marriage had been valid, providing grounds for the annulment.