A thick sauce in classic French cuisine that consists of a reduction of vinegar, wine, sugar, and sometimes fruit that has been caramelized over heat.

A gastrique is employed to round out flavors in a dish, and is either served with meat or seafood, or used as a base for other sauces, such as one with tomatoes.

It can also be used to add complexity to fruit-forward cocktails.

Source: Flickr User finna dat