Decantation
The separation of a liquid from a settled solid or a denser immiscible liquid by carefully pouring off the upper layer.
Unit
Dimensionless
In depth
Decantation relies solely on a density difference and gravity, so it is quick and needs no apparatus, but it never removes the last traces of the lower phase. It is usually a preliminary step, with filtration or centrifugation completing the separation.
Examples in the real world
Pouring water off settled sand, or separating an oil layer from water after allowing an emulsion to break.