Definition:

To reduce is to cook away a certain portion of a liquid, for example, by simmering in a saucepan until some of the liquid evaporates. The remaining liquid is more concentrated in flavor and often has a slightly thicker, more unctuous texture. Reducing is often done for sauces to make them thicker or richer in flavor. Typically the recipe indicates how much the liquid should be reduced, such as by half.

Examples: Reduce the chicken stock mixture by half.