The Collaborative International Dictionary of English v.0.48:
Roux \Roux\, n. [F. beurre roux brown butter.] (Cookery) A thickening, made of flour, for soups and gravies. [1913 Webster]WordNet (r) 3.0 (2006):
roux n 1: a mixture of fat and flour heated and used as a basis for sauces