The ounce ( abbreviated : oz , the old Italian word onza , now spelled oncia ) is a unit of mass in a number of different systems , including various systems of mass that form part of the imperial and United States customary systems . The ounce ( abbreviated : oz , the old Italian word onza , now spelled oncia ; apothecary symbol : â ) is a unit of mass with several definitions , the most commonly used of which are equal to approximately 28 grams . The ounce is used in a number of different systems , including various systems of mass that form part of the imperial and United States customary systems .