The Collaborative International Dictionary of English v.0.48:

Dowager \Dow"a*ger\, n. [OF. douagiere, fr. douage dower. See
   Dower.]
   1. (Eng. Law) A widow endowed, or having a jointure; a widow
      who either enjoys a dower from her deceased husband, or
      has property of her own brought by her to her husband on
      marriage, and settled on her after his decease. --Blount.
      --Burrill.
      [1913 Webster]

   2. A title given in England to a widow, to distinguish her
      from the wife of her husband's heir bearing the same name;
      -- chiefly applied to widows of personages of rank.
      [1913 Webster]

            With prudes for proctors, dowagers for deans.
                                                  --Tennyson.
      [1913 Webster]

   Queen dowager, the widow of a king.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dowager
    n 1: a widow holding property received from her deceased husband