lagoon

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

• noun A shallow body of water, especially one separated from a sea by sandbars or coral reefs.
• noun A shallow artificial pond used for treating or storing liquid waste material or for collecting flood waters.

from The Century Dictionary.

• noun An area of shallow water, or even of marshy land, bordering on the sea, and usually separated from the region of deeper water outside by a belt of sand or of sand-dunes, more or less changeable in position.
• noun With reference to Tuscany and some other parts of Italy, the basin of a hot spring, especially one from which borax is obtained: from the Italian use of lagone in this sense.
• noun In occasional use, the area of still water inclosed within an atoll, which is often called a lagoon island. See atoll.

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

• noun A shallow sound, channel, pond, or lake, especially one into which the sea flows.
• noun A lake in a coral island, often occupying a large portion of its area, and usually communicating with the sea. See Atoll.
• noun a coral island consisting of a narrow reef encircling a lagoon.

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

• noun A shallow body of water separated from deeper sea by a bar.

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

• noun a body of water cut off from a larger body by a reef of sand or coral