Source : Webster's Revised Unabridged Dictionary (1913)

Estuary \Es"tu*a*ry\, a.
   Belonging to, or formed in, an estuary; as, estuary strata.
   --Lyell.

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.

Source : WordNet®

estuary
     n : the wide part of a river where it nears the sea; fresh and
         salt water mix