The term splash is typically used as a bar tender measurement for adding a very small amount of liquid to a drink, approximately 1/8 ounce.
This term is not uncommon in U.S. recipes either. In cooking, clearly the ingredient is not overly crucial if it is below a standard U.S. measurement such as a 1/16 of a teaspoon in which case, a "splash" should be enough. You'd usually see it in reference to something like "a splash of pepper sauce" or perhaps a recipe might read "a splash of wine" for instructions on deglazing a pan.