Just as Baron So-and-so is a title but the person holding it is always referred to as Lord So-and-so, not Baron, so a baroness is referred to as Lady So-and-so.

The exception is women who are created life peer. They are called e.g. Baroness Jay, Baroness Thatcher, to make it clear they hold the title in their own right, because the style "Lady So-and-so" does not make this clear.

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.

 

© Webster 1913.