Lady was an aristocratic title. The male equivalent was "Lord".
Lady Jennifer Buckingham was a nurse and ambulance driver who served in the Women's Volunteer Reserve during World War I. She was abducted by the War Lords and hypnotised into participating in their simulated version of the war. (TV: The War Games)
Lady Peinforte was a cavalier of Windsor in 1638, as well as a sorceress and poisoner. She threatened to reveal the Seventh Doctor's secrets if he would not give her the Nemesis bow. He refused and gave it to the Cyber-Leader. The Cyber-Leader expressed no interest in knowing the secrets Lady Peinforte offered to tell him, driving her mad. Just before the Nemesis prepared to lift off from Earth as the Doctor had programmed it, Lady Peinforte jumped onto the statue and merged with it. In space, the Nemesis exploded, destroying the entire Cyber-Fleet and Lady Peinforte as well. (TV: Silver Nemesis)
Lady Sylvia Thaw helped Richard Lazarus develop his rejuvenating device. She was also Mr Saxon's representative on the project. When Lazarus mutated after his rejuvenation went faulty, he drained the life out of her as he needed the energy for the mutation to stabilise. (TV: The Lazarus Experiment)