What is the definition of "mass"?

Mass is the amount of matter in an object, and it is different from weight. Mass is also defined as the measure of an object's inertia.

Defining the concept of mass can be difficult, says UCLA, because scientists also need to define abstract concepts such as matter. In the metric system, amounts of matter, or mass, are measured in grams and kilograms, while in the United States, mass is technically measured in a unit called the slug. People often confuse mass and weight, but weight is dependent on the gravity in a given setting, while mass remains constant regardless of its setting.