absorption, soaking up - (chemistry) a process in which one substance permeates another; a fluid permeates or is dissolved by a liquid or solid

adsorption, surface assimilation - the accumulation of molecules of a gas to form a thin film on the surface of a solid

natural action, natural process, activity, action - a process existing in or produced by nature (rather than by the intent of human beings); "the action of natural forces"; "volcanic activity"