Source : Webster's Revised Unabridged Dictionary (1913)

Baroness \Bar"on*ess\, n.
   A baron's wife; also, a lady who holds the baronial title in
   her own right; as, the Baroness Burdett-Coutts.

Source : WordNet®

baroness
     n : a noblewoman who holds the rank of baron or who is the wife
         or widow of a baron