In cooking, reduction is the process of thickening

Thickening

In cooking, thickening is the process of increasing the viscosity of a liquid either by reduction, or by the addition of a thickening agent, typically containing starch....

 and intensifying the flavor of a liquid mixture such as a soup

Soup

Soup is a generally warm food that is made by combining ingredients such as meat and vegetables with stock, juice, water, or another liquid. Hot soups are additionally characterized by boiling solid ingredients in liquids in a pot until the flavors are extracted, forming a broth.Traditionally,...

, sauce

Sauce

In cooking, a sauce is liquid, creaming or semi-solid food served on or used in preparing other foods. Sauces are not normally consumed by themselves; they add flavor, moisture, and visual appeal to another dish. Sauce is a French word taken from the Latin salsus, meaning salted...

, wine

Wine

Wine is an alcoholic beverage, made of fermented fruit juice, usually from grapes. The natural chemical balance of grapes lets them ferment without the addition of sugars, acids, enzymes, or other nutrients. Grape wine is produced by fermenting crushed grapes using various types of yeast. Yeast...

, or juice

Juice

Juice is the liquid that is naturally contained in fruit or vegetable tissue.Juice is prepared by mechanically squeezing or macerating fruit or vegetable flesh without the application of heat or solvents. For example, orange juice is the liquid extract of the fruit of the orange tree...

 by boiling

Boiling

Boiling is the rapid vaporization of a liquid, which occurs when a liquid is heated to its boiling point, the temperature at which the vapor pressure of the liquid is equal to the pressure exerted on the liquid by the surrounding environmental pressure. While below the boiling point a liquid...

.
Reduction is performed by boiling liquid (whether stock

Stock (food)

Stock is a flavoured water preparation. It forms the basis of many dishes, particularly soups and sauces.- Preparation :Stock is made by simmering various ingredients in water, including some or all of the following...

, wine, whiskey, vinegar, or sauce mixture) rapidly and usually without a lid (enabling the vapor to escape more easily) until the volume desired is reached by evaporation. Since each components of the mixture evaporate at slightly different temperatures and the goal of reduction is to drive away those with lowest points of evaporation, it is – in a way – a form of distillation

Distillation

Distillation is a method of separating mixtures based on differences in volatilities of components in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....

.

While reduction does concentrate the flavors left in the pan, extended cooking can drive away volatile flavor compounds or even juice self to the meal, leaving behind dryer, less juicy food without essential flavours.

Examples

Common preparations involving reductions include

• Consommé
  Consommé
  In cooking, a consommé is a type of clear soup made from richly flavored stock or bouillon that has been clarified usually through a fining process involving egg protein. It usually requires an advanced knowledge of cooking and past experience to create a high quality consommé...
  
  s, reduced and clarified stocks
  Stock (food)
  Stock is a flavoured water preparation. It forms the basis of many dishes, particularly soups and sauces.- Preparation :Stock is made by simmering various ingredients in water, including some or all of the following...
  
  
• Gravies
  Gravy
  Gravy is a sauce made often from the juices that run naturally from meat or vegetables during cooking. In North America the term can refer to a wider variety of sauces and gravy is often thicker than in Britain...
  
  
• Gastrique
  Gastrique
  Gastrique is caramelized sugar, deglazed with vinegar, used as a flavoring for sauces.To this a little fond may be added. It is used to flavor sauces such as tomato sauce, savory fruit sauces, and other sweet and sour sauces such as the classic orange sauce for duck à l'orange.Nowadays, the term ...
  
  s, sauces involving both acidic and sweet components
• Pan sauce
  Pan sauce
  A pan sauce is a type of sauce that is often made to accompany sauteed meats.Generally speaking, a pan sauce is made by sauteing a meat at high temperature in a skillet, removing the meat, pouring off any excess fat, deglazing the pan with a liquid such as stock or wine, and incorporating the...
  
  s
• Syrup
  Syrup
  In cooking, a syrup is a thick, viscous liquid consisting primarily of a solution of sugar in water, containing a large amount of dissolved sugars but showing little tendency to deposit crystals...
  
  s