Estuary \Es"tu*a*ry\, n.; pl. {Estuaries}. [L. aestuarium, from aestuare to surge. See {Estuate}.] [Written also {[ae]stuary}.] 1. A place where water boils up; a spring that wells forth. [Obs.] --Boyle. 2. A passage, as the mouth of a river or lake, where the tide meets the current; an arm of the sea; a frith. it to the sea was often by long and wide estuaries. --Dana.