The term Lady is used as an honorific for woman to indicate that they or their close relative has rank, often being a peer. In modern England, a woman is called a Lady if she is a marchioness, countess, viscountess, baroness or baronetess, if their husband is a knight, baron or baronet or if their father is a duke, marquis or earl. It can also be used for the wife of the younger son of a marquis or earl.
Hence, you could refer to the Baroness of Outerkeep, Lady Catherine or the wife of Earl Snowdon can be referred to as Lady Snowdon.
A Lady of the SCA