Mass
Definition
How much matter an object contains. Mass is what makes things hard to push around and what gives them weight in a gravitational field.
Symbol: · Unit: kilogram (kg)
Technical definition
A measure of an object's inertia — its resistance to acceleration — and of its coupling to gravitational fields. In Newtonian physics mass is conserved and constant; in relativity it is interchangeable with energy via E = mc².
Example
A bowling ball (about 7 kg) is much harder to accelerate than a tennis ball (about 0.06 kg) — the same push produces a far smaller acceleration on the bowling ball, because acceleration equals force divided by mass.
Mass is not weight. Mass is the amount of matter and never changes when you travel; weight is the gravitational force on that mass and changes with location. You have the same mass on the Moon, but you weigh about one-sixth as much there. Scales measure weight and convert it; balances compare mass.