extensive property
Take a glass of water and a swimming pool of the same water. The pool obviously contains far more water — more mass, more volume, more total energy. Any property that grows in step with how much stuff you have is called an extensive property. Double the amount of material and you double the property.
Formally, an extensive property depends on the size or amount of the system. Mass, volume, total energy, enthalpy, entropy, and heat capacity are all extensive: each adds up when you combine two portions. If you put two identical cups of coffee together, the combined mass and volume are twice as large.
Knowing whether a property is extensive matters whenever you scale a process up or down — say from a test tube to an industrial vat. Extensive properties scale with the batch; you cannot quote them sensibly without saying how much you have. This is exactly why chemists so often divide an extensive property by the amount, turning it into a size-independent intensive one.
Two litres of milk weigh twice as much as one litre and hold twice the energy, but both are at exactly the same temperature in the fridge — mass and energy are extensive, temperature is not.
Extensive properties add up when you combine portions; intensive ones do not.
Divide one extensive property by another and you often get an intensive one: mass divided by volume gives density, which no longer cares how much you have.