Sour mix is a mixer that is yellow-green in color and is used in many cocktails. It is made from approximately equal parts lemon and/or lime juice and simple syrup and shaken vigorously with ice. This produces a pearly-white liquid with a pronounced flavor.
Optionally, egg whites may be added to make the liquid slightly foamy.
Sour mix can be mixed with liquor(s) to make a sour drink; most common are vodka sour (vodka) and whiskey sour (whiskey).
Pre-mixed versions are available, and are in use in many bars. These typically consist of a powder which must be rehydrated by adding water prior to use.