Butter cooked until it's a pale brown color, akin to that of hazelnuts. The French term is beurre noisette ("hazelnut butter"). Brown butter is used to dress myriad foods including vegetables, fish, meat and pasta.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.