a sustained phenomenon or one marked by gradual changes through a series of states

(of e.g. volcanos) erupting or liable to erupt

(of the sun) characterized by an increased occurrence of sunspots and flares and radio emissions

the spread of a group of organisms into new habitats

(physics) the process in which incident radiated energy is retained without reflection or transmission on passing through a medium

the process of becoming acid or being converted into an acid

(thermodynamics) any process that occurs without gain or loss of heat

the process of exposing to air (so as to purify)