Demi-glace is a dark brown sauce used in French cooking. It is traditionally made by combining equal amounts of Espagnole and brown stock.
If no meat type is mentioned, demi-glace is always assumed to be made with veal. Beef demi-glace (demi-glace au boeuf) and chicken demi-glace demi-glace au poulet) variations do exist.
When demi-glace is applied as an adjective (e.g. "Roast beef demi-glace") it implies that the dish has been glazed or sauced with a portion of demi-glace sauce.