A property of a physical system is said to be extensive if it is dependent on the amount of stuff present in the system1. "Stuff" can be any quantity of interest, like number of atoms, mass, electric charge, etc. For instance, mass and volume are extensive because they depend on the amount of atoms in the object, but mass density is not. Another definition is "any property whose magnitude sums for subsystems"2.
The opposite of an extensive property is an intensive property.
These properties can be seen as being attached to a specific object, like a specific weight, particle or star. Generally speaking, a property can be reasonably described as extensive when the object is far larger than its constituents (usually atoms) and no other more specific conditions are met. These include
- the system being too small compared to its constituents
- being submerged in a nonuniform potential
- the presence of long-range, non-negligible internal forces
- the surface-to-volume ratio is heavily slanted in favor of the surface
These do occur even in real systems. For instance, the volume of a star is not cleanly extensive because the gravitational interaction between atoms tends to counteract the increase in size due to adding more atoms, which makes the volume function non-linear.
