Sous vide is French for "under vacuum". It is a method of cooking in which a food is sealed in a plastic bag and cooked for a long period of time at a low temperature. This process prevents the evaporative loss of flavor volatiles (the chemicals in food that give it taste) and moisture.
Chicken, vacuum sealed, flavored with orange.
The sealed bag of food is placed into a water bath--usually temperature controlled by an electric heating element--and left to cook. By cooking in a liquid that is of uniform temperature, the bags contents are raised to the temperature of the water. Over cooking, in the traditional sense, is impossible.
**Foods can become mushy if left in the water bath for too long.
The most commonly used example is that of a steak. When cooking a steak (aiming for medium-rare), you use an extremely hot (usually around 600 degrees fahrenheit) surface to create a caramelized crust and bring the center of the steak of 120 degrees fahrenheit. However, this technique invariably leads to a steak that is well done around the edges, medium, and finally medium rare at the center. By using the sous vide method, the steak is placed into a 120 degree F water bath and cooked until the entire steak is 120 F, from edge to center.
Below are three images. The first is a steak vacuum sealed. The second is a steak cooked sous vide. The third, for comparison, is a steak cooked to medium rare using a traditional grill and broiler (as you can see, it is not uniformly cooked).
In restaurants, sous vide is done using an immersion circulator, shown below.
This technique is usually used on meats (steak, chicken), fish and fruits (poached pear).
Example: For a sweet example, custard base can be vacuum sealed into a cylindrical mold and cooked sous vide. Here, Meyer lemon custard (the yellow disks) is paired with a tarragon liquid gel (the green sauce), yuzu caramel (yellow sauce), and black pepper blood orange sorbet.