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

Moby Thesaurus II by Grady Ward, 1.0:

64 Moby Thesaurus words for "estuary":
   arm, armlet, avenue, bay, bayou, belt, bight, blowhole, boca,
   channel, chute, cove, creek, debouch, door, egress, emunctory,
   escape, euripus, exhaust, exit, fjord, floodgate, flume, frith,
   gulf, gut, harbor, inlet, kyle, loch, loophole, mouth, narrow,
   narrow seas, narrows, natural harbor, opening, out, outcome,
   outfall, outgate, outgo, outlet, pore, port, reach, road, roads,
   roadstead, sally port, sluice, sound, spiracle, spout, strait,
   straits, tap, vent, ventage, venthole, vomitory, way out, weir