Princess Mother

A queen dowager, dowager queen or queen mother (compare: princess dowager, dowager princess or princess mother) is a title or status generally held by the widow of a king. In the case of the widow of an emperor, the title of empress dowager is used. Its full meaning is clear from the two words from which it is composed: queen indicates someone who served as queen consort (i.e. wife of a king), while dowager indicates a woman who holds the title from her deceased husband (a queen who rules in her own right and not due to marriage to a king is a queen regnant).

A queen mother is a dowager queen who is the mother of the reigning monarch. Currently there are four queens dowager: Kesang Choden of Bhutan (who is the only living queen grandmother worldwide), Norodom Monineath of Cambodia (who is also queen mother), Lisa Najeeb Halaby (Noor Al'Hussein), and Sirikit Kitiyakara of Thailand. Queen Ratna of Nepal was queen dowager and queen mother until the abolition of the Nepalese monarchy in 2008.

A queen mother is a particular type of queen dowager who is simultaneously a former queen consort and the mother of the current monarch. Therefore, every queen mother is by definition also a queen dowager. However, not all queens dowager are queens mother; they may have a relation other than mother to the reigning monarch, such as aunt or grandmother. For example, Mary, Queen of Scots, was queen dowager of France after the death of her husband Francis II, to whom she bore no children. Similarly, Adelaide of Saxe-Meiningen was queen dowager after her husband William IV was succeeded by his niece Victoria.

...
Wikipedia