Reduce

To reduce is to cook a liquid uncovered until some of its water evaporates, leaving less of it, with a stronger flavour and usually a thicker texture.

It is how a splash of wine becomes a pan sauce, a thin stock becomes a glossy gravy, and a watery tomato sauce turns rich. Recipes often say how far to go: "reduce by half" means until about half the volume is left. A clean line on the side of the pan, or a quick look at how deep the liquid sits, helps you judge it.

A wide pan reduces faster than a deep one, because more of the surface is exposed. A brisk simmer works for most things. A hard boil is fine for wine or stock on their own but can make sauces with flour or eggs catch or split.

Season at the end. Salt doesn't evaporate, so a sauce that tasted right at the start can be too salty once reduced.

The word is the same in British and American English.