Absorption

Absorption is when something takes in another substance. It is a physical or chemical phenomenon or process. Atoms, molecules, or ions enter in the inner part (called "bulk") of a gas, liquid, or solid material. 
This is different 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: