The Marquis of Longtown, also referred to as Lord Longtown, is a character mentioned in Northanger Abbey, and never appears in the book. He is a friend of General Tilney's. The Tilney family go to stay with him for a fortnight due to an unexpected engagement. The general brings his entire family to Lord Longtown's estate, which means Catherine Morland must return to Fullerton and her parents.
Longtown holds the title of "marquis" or "marquess", which means he is only one rank down from a duke. This does mean, however, that he is called "Lord Longtown" in reference style.