Complex sentence: In physics, mass is a property of a physical body which determines the strength of its mutual gravitational attraction to other bodies, its resistance to being accelerated by a force, and in the theory of relativity gives the mass--energy content of a system.
Split into simpler sentences: In physics, mass is a property of a physical body. It is determined by the strength of its mutual gravitational attraction to other bodies, its resistance to being accelerated by a force, and in the theory of relativity gives the mass--energy content of a system.