Washington, D.C. is the capital of the United States of America, located along the Potomac River on the country's East Coast.
In Realms of the Haunting, the city is referred to in one of the letters taht we find inside the Mansion's Study, allowing us to draw some more information on the characters' backgrounds.
According to the fourth letter, Rebecca must have dwelled in Washington around the year 1930, among others frequenting the University and the National Geographic Society in Washington, in search of certain manuscripts and books required by Florentine.