Béchamel

A white sauce built from a butter-and-flour roux into which warm milk is whisked.

What it means in a kitchen

Béchamel is one of the French mother sauces. The roux ratio is the foundation — typically 1:1 butter to flour by weight. The temperature differential between hot roux and cold milk is what causes lumps; warm the milk first and whisk constantly and the sauce stays smooth.