Solution
A homogeneous mixture in which one or more substances are dispersed uniformly at the molecular or ionic level in another.
Unit
Dimensionless
In depth
A solution has a single phase, particles smaller than about 1 nm, and a composition that can be varied continuously within the solubility limit. It never settles on standing and cannot be separated by filtration, only by techniques that exploit differences in volatility or affinity.
Examples in the real world
Brine, brass (a solid solution of zinc in copper), and air as a gaseous solution of oxygen in nitrogen.