The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. A passage, as the mouth of a river or lake, where the tide
      meets the current; an arm of the sea; a frith.
      [1913 Webster]

            it to the sea was often by long and wide estuaries.
                                                  --Dana.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Estuary \Es"tu*a*ry\, a.
   Belonging to, or formed in, an estuary; as, estuary strata.
   --Lyell.
   [1913 Webster]

WordNet (r) 3.0 (2006):

estuary
    n 1: the wide part of a river where it nears the sea; fresh and
         salt water mix