Beurre blanc

A warm emulsion sauce of butter and a wine-vinegar reduction, traditionally served with fish.

What it means in a kitchen

A wine-and-vinegar reduction is built down to a syrup, then cold butter is whisked in piece by piece off direct heat. The acid stabilizes the emulsion; the butter carries flavor. It holds best somewhere between roughly 50 and 60°C — outside that window it tends to break.