From WordNet (r) 2.0 (August 2003) [wn]:
baroness
n : a noblewoman who holds the rank of baron or who is the wife
or widow of a baron
From The Collaborative International Dictionary of English v.0.48 [gcide]:
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.
[1913 Webster]