The title Lord or Lady is a term usually used to address individuals holding certain positions, such as noblemen. The term is also used as an alternate term for the Christian "God" (DS9: "Far Beyond the Stars"), and as a general honorific used by the Androids of Mudd for those they serve. (TOS: "I, Mudd").
The title was historically used on Earth for noblemen in some Nation states. Some figures holding the title of lord on Earth were Lord Byron, Lord Halifax, Lord Nelson. Additionally, a fictional 19th century Lord Burleigh appeared in Kathryn Janeway's Holonovel Janeway Lambda one.
Troyian ambassador Petri was referred to as Lord. (TOS: "Elaan of Troyius"). An insane Garth of Izar insisted on being called "Lord Garth" while he temporarily took control of the Asylum on Elba II (TOS: "Whom Gods Destroy"). Klingon commander Kruge was also referred to by his crew, and Valkris as "my Lord". (Star Trek III: The Search for Spock)