In physics, mass is a fundamental property of matter that measures the amount of resistance to acceleration an object has when a force is applied. It’s a scalar quantity that’s always positive or zero, and it’s expressed in units of kilograms or grams.

The standard unit of mass is the kilogram (kg), which is defined as the mass of a platinum-iridium cylinder kept at the International Bureau of Weights and Measures in France. Mass can be measured using a balance or a scale, and it’s conserved in physical systems, meaning that the total mass of a system remains the same before and after a physical change or chemical reaction.

Mass is one of the most important concepts in physics because it’s involved in many fundamental equations and laws, including Newton's Second Law (force is proportional to mass times acceleration), the Law of Conservation of Mass (mass can’t be created or destroyed in chemical reactions), and Einstein’s famous equation E=mc^2 (which relates mass and energy).