A measure of how much matter is in an object.
This gold bar is quite small but has a mass of 1 kilogram (about 2.2 pounds), so it contains a lot of matter.
Mass is commonly measured by how much something weighs. But weight can change depending on where you are (such as on the moon) while the mass stays the same.
Mass is measured in grams, kilograms and, tonnes (Metric) or ounces and pounds (US units)