Intensive quantity
A property independent of system size — temperature and pressure are the same whether you take half the gas or all of it.
Definition
An intensive quantity is one that does not change when the size of the system changes: split a uniform body in two and each half has the same temperature, pressure, density, and chemical potential as the whole. Intensive quantities are local properties; they can vary from point to point but do not add up when systems are combined.
Intensive variables stand opposite extensive ones (mass, volume, energy, entropy), which scale with system size. Every intensive variable in thermodynamics is conjugate to an extensive one and can be obtained as a derivative of energy with respect to it: temperature T = ∂U/∂S, pressure P = −∂U/∂V, chemical potential μ = ∂U/∂N. Equality of an intensive variable across two systems in contact is the condition for the corresponding equilibrium — equal temperature for thermal equilibrium, equal pressure for mechanical equilibrium.
Because the ratio of two extensive quantities is intensive, densities and per-particle quantities are the natural intensive descriptors of bulk matter.
History
Like its extensive counterpart, the concept was made systematic in Gibbs's formulation of thermodynamics and the theory of phase equilibria in the 1870s.