The quantity measured in kilograms that describes how much matter an object contains is called mass. In classical physics, mass also measures an object's resistance to acceleration: for the same applied force, a larger-mass object accelerates less. Unlike weight, mass does not depend on the strength of the local gravitational field.
An object's mass is therefore essentially the same on Earth, the Moon, or in deep space, while its weight changes because gravity changes. A one-kilogram object has a weight of about 9.81 newtons at Earth's surface, but much less on the Moon. The kilogram is the SI base unit of mass, and since 2019 it has been defined using the fixed value of Planck's constant rather than a physical metal artifact.
Modern relativity adds an important refinement: physicists usually refer to invariant or rest mass, which is independent of the observer's motion. Mass is also related to energy through Einstein's equation E = mc². Volume measures how much space an object occupies, density is mass per unit volume, and weight is a gravitational force—not the same quantity as mass.