Reduction
Boiling or simmering a liquid so that water evaporates, leaving it thicker and stronger in flavour.
Also: reduce
To reduce a liquid is to cook it uncovered until some of its water has boiled away. What remains is thicker and tastes more concentrated. Cooks reduce stock, wine, cream, vinegar and fruit juice to make sauces and glazes without adding starch. A wide pan speeds the process because more surface is exposed. Salt does not evaporate, so a liquid is usually seasoned after reducing, not before. Milk reduced slowly becomes the base of many Indian sweets.