How do you define mass?

In physics, mass is defined as a quantity without dimensions that represents the amount of matter in an object or particle. It can also be defined as a property of a physical body which determines the strength of its mutual gravitational attraction to other bodies.

Mass is measured by figuring out the degree to which a particle or object resists a change in its speed or direction when a force is applied. Isaac Newton stated that a stationary mass remains stationary, and a mass in motion at a constant speed and in a constant direction maintains that state of motion unless acted on by an outside force.