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

Moby Thesaurus II by Grady Ward, 1.0:

57 Moby Thesaurus words for "dowager":
   Eve, Frau, abbess, beldam, chatelaine, crone, dame,
   daughter of Eve, distaff, domina, donna, femme, first lady, frow,
   frump, gentlewoman, girl, goodwife, governess, grandam,
   grande dame, grandmother, granny, great lady, hag, homemaker,
   housewife, lady, lass, madam, matriarch, matron, milady, mistress,
   mother superior, old battle-ax, old dame, old girl, old granny,
   old lady, old trot, old wife, old woman, queen dowager, relict,
   squaw, trot, vrouw, wahine, war-horse, weaker vessel, widow,
   widow woman, widower, widowman, witch, woman



Bouvier's Law Dictionary, Revised 6th Ed (1856):

DOWAGER. A widow endowed; one who has a jointure.
     2. In England, this is a title or addition given to the widows of
princes, dukes, earls, and other noblemen.