estuary

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The part of the wide lower course of a river where its current is met by the tides.
• noun An arm of the sea that extends inland to meet the mouth of a river.

from The Century Dictionary.

• noun An arm or inlet of the sea, particularly one that is covered by water only at high tide.
• noun That part of the mouth or lower course of a river flowing into the sea which is subject to tides; specifically, an enlargement of a river channel toward its mouth in which the movement of the tides is very prominent.
• noun A place where water boils up.
• Belonging to or formed in an estuary: as, estuary strata.

from the GNU version of the Collaborative International Dictionary of English.

• adjective Belonging to, or formed in, an estuary.
• noun obsolete A place where water boils up; a spring that wells forth.
• noun A passage, as the mouth of a river or lake, where the tide meets the current; an arm of the sea; a frith.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Coastal water body where ocean tides and river water merge.
• noun An ocean inlet also fed by fresh river water.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the wide part of a river where it nears the sea; fresh and salt water mix