Mass is a fundamental property of matter. Matter is anything that has mass and takes up space. The amount of matter in an object is measured by its mass. The more matter an object has, the more massive it is. Mass is also a measure of an object's inertia, which is its resistance to changes in its motion. The more massive an object is, the more inertia it has.

Mass is an important concept in physics because it is used to calculate other physical quantities, such as force, momentum, and energy. Mass is also used to determine the gravitational force between objects.

The mass of an object can be measured using a balance. A balance compares the mass of an object to the mass of a known object. The known object is called the reference mass. The mass of the object being measured is determined by balancing it against the reference mass on the balance.

Mass is a scalar quantity, which means that it has only magnitude and no direction. The SI unit of mass is the kilogram (kg).