The ounce (abbreviated: oz, the old Italian word "onza", now spelled "oncia") is a unit of mass or of weight in a number of different systems, including various systems of mass that form part of the imperial and United States customary systems. Its size can vary from system to system. The most commonly used ounces used today are the international avoirdupois ounce of about 28.35 grams, and the international troy ounce of about 31.1 grams.