What is "beurre blanc"

Beurre blanc — literally translated from French as "white butter" — is a hot emulsified butter sauce made with a reduction of vinegar and/or white wine (normally Muscadet) and grey shallots into which cold, whole butter is blended off the heat to prevent separation. The small amounts of lecithin and other emulsifiers naturally found in butter are used to form an oil-in-water emulsion. Although similar to hollandaise in concept, it is considered neither a classic leading nor compound sauce. This sauce originates in Loire Valley cuisine.