In popular usage, the noun dowager may refer to any elderly widow, especially one of both wealth and dignity.
In the United Kingdom, the widow of a peer may continue to use the style she had during her husband's lifetime, e.g. "Countess of Loamshire", provided that his successor, if any, has no wife to bear the plain title. Otherwise she more properly prefixes either her forename or the word Dowager, e.g. "Jane, Countess of Loamshire" or "Dowager Countess of Loamshire". (In any case, she would continue to be called "Lady Loamshire".)
The term queen dowager is used in the United Kingdom and several other countries for the widow of a king. However, if the dowager is the mother of the current monarch, she is more often known as the queen mother.
Empress dowager is the title given to the mother or widow of a Chinese, Japanese, Korean or Vietnamese emperor. A notable example is Chinese Empress Dowager Cixi, who effectively ruled China for over 47 years.