1. The division and parting of materials into their constituent parts. Separation is used for purification by removing contaminants or for enrichment. In some cases, separation may involve removing single components or groups of components from a mixture. Examples of physical and chemical separation processes include chromatography, distillation, evaporation, drying, electrolysis, desorption, and gravity. The petrochemical refining of crude oil is an industrial process used to separate the complex mixture of hydrocarbons into valuable components largely through distillation. The choice of separation is based on the chemical and physical properties of the materials such as chemical affinity with other components, size, shape, and density.

2. The phenomenon of fluid streamlines changing direction due to changes in boundary shape as a result of fluid inertia or velocity distribution near the boundary surface.