Queen Dowager is a title traditionally given to the widow of a male monarch during the reign(s) of his successor(s), especially if the next monarch is not a child of the previous one (in which case Queen Mother might be used). If there are multiple widows of kings, then the senior takes to the title.
For example, after the death of William IV in 1837, until her death in 1849, his widow, Adelaide of Saxe-Meiningen stopped being Her Majesty The Queen and became Her Majesty The Queen Dowager.