A phase of matter is when all the physical properties within a material are uniform. At a given phase of matter a material will have the same density and refractive index. For example, a cube of ice (solid water) is at one phase, liquid water is in another phase and water vapor is the third phase.

States of matter are classifications of the distinct phases of matter based on their large-scale properties. For example, a solid phase is the state in which the substance maintains a fixed volume and a fixed shape, whereas the liquid phase is one where the substance can change to take the shape of its container.