In science, mass is the amount of matter an object has. Newton's second law says that mass is the property of matter that determines the force required to impart a given acceleration to an object.
Mass will always be a fixed quantity. Mass is not the same thing as weight. If one takes something with the mass of 5 kilograms to the moon, the mass does not change, even though the weight is only 1/6 of what it is on the Earth. The standard measurement used for this fixed quantity is always kilograms. The kilogram is an internationally recognized measurement for mass.