In physical science, mass is a measure of the number of atoms in an object combined with the density of those atoms. The mass of an object determines how that object responds to forces such as gravity and friction. The typical unit of measure for mass is kilograms.
Mass is a fundamental property of matter, which is anything that has mass and takes up space. Central to the concept of mass is the concept of inertia. All physical objects resist changes to their states of motion. Inertia is the name of this resistance. Mass is a quantity that is solely dependent on an object or system's resistance to change. Objects of higher mass have higher resistance, or inertia.
Mass is not necessarily related to size. For example, a large balloon can have very little mass, and a lead bullet can be very small but have a great deal of mass. Mass is not the same as weight, but on earth, they function in similar ways: objects with more mass weigh more than objects with less mass. This is due to the experience of gravity on earth. An object's weight changes under different gravitational forces, but its mass does not change.