estuary estuary /ˈes.tjʊə.ri/ aep /-tu.er.i/ noun [C] ► F0 the wide part of a river at the place where it joins the sea: » the Thames estuary» the Rance estuaryThesaurus+: ↑Parts of watercoursesestuarine /-tjʊə.riːn/ aep /-tjʊr.iːn/ adjectiveSPECIALIZED ► F0» estuarine speciesThesaurus+: ↑Parts of watercourses