Topping, or Topping off is the act of finishing a drink by adding a user-determined amount of a liqueur, juice, etc. to a cocktail. Generally, it is the final step of preparation prior to garnishing.
Usually drinks are topped with Tonic water, Soda water, Energy drink, Cola, or whatever is determined by the recipe.
An alternate form of the term is Fill.
Note[]
There is a slight difference in proper nomenclature between "topping" and "topping off"; "topping" is done prior to serving, as part of the recipe and preparation, while "topping off" is done after serving the cocktail, either to compensate for inadequate amount or spillage or after the cocktail has been partly consumed.
"Topping off" is most commonly done with unmixed drinks, either neat or On the Rocks.
The term is also commonly (and erroneously) used to indicate garnishing a drink.