Demi Glace

Demi-glace is a rich brown sauce in French cuisine used by itself or as a base for other sauces. The term comes from the French word glace, which, when used in reference to a sauce, means "icing" or "glaze". It is traditionally made by combining one part espagnole sauce and one part brown stock. The sauce is then reduced by half, strained of any leftover impurities, and finished with a sherry wine... (Wikipedia)