Mass is a measure of how much matter is in an object and how much it resists acceleration; weight is how heavy that object is because of gravity acting on it. A memory hook: your mass stays the same on the Moon, but your weight drops, because there is less gravity pulling on the same matter.
The core difference
Mass measures the amount of matter in an object, while weight measures how strongly gravity pulls on that matter.
- mass — how much matter an object contains, and its resistance to acceleration: The rocket's mass decreases as it burns fuel.
- weight — how heavy someone or something is: The doctor asked about his recent change in weight.
How to tell them apart
Mass comes from the Latin massa, meaning a lump or dough, while weight comes from an Old English root meaning to move or carry, related to "weigh". That history hints at the difference in everyday and scientific use: mass is about the raw amount of stuff, while weight is about the pull that stuff feels.
The confusion shows up most in physics classes and in everyday speech, where people say "weight" when they mean mass, such as stepping on bathroom scales. Outside physics, mass also has everyday uses, from a mass of dark clouds to a mass recall of a faulty product, while weight can mean a physical object used for exercise, as in lifting weights at the gym.
