Absorption is a condition in which something takes in another substance. It is a physical or chemical phenomenon or process, in which atoms, molecules, or ions enter in the inner part (called "bulk") of a gas, liquid, or solid material.
This is a different process from adsorption. Molecules undergoing absorption are going deep into the volume of the material, instead in the case of adsorption molecules stop at the surface of the material. A more general term is sorption, which covers absorption, adsorption, and ion exchange.
Other meanings of "absorption" are: