The ounce is a unit of mass and weight within the imperial and US customary systems of measurement. Historically, its definition varied based on the substance being measured, particularly when referring to body weight versus the mass of materials. Although numerous specialized definitions exist, the concept of the ounce is most commonly used to quantify relatively small quantities of mass, serving as a subunit of the pound. Within the context of physical measurement, the definition is critical for maintaining consistency across different fields of study, including chemistry and basic material science.