vee molekulide sisestamine mingi osakese koostisse, millega kas kaasneb või ei kaasne seal leiduvate aatomite või aatomirühmade asendamine (nt vee molekulide sisestamine anorgaanilise kompleksühendi sisesfääri), the incorporation of one or more integral molecules of water into another species with or without displacement of one or more other atoms or groups. For example, the incorporation of water into the innerligand sphere of an inorganic complex is an aquation reaction.[ PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1086]