A shallow body of water separated from deeper sea by a bar.
"The small boat drifted gently across the calm lagoon, which was protected from the rough ocean waves by a narrow sandbar."
In plain English: A lagoon is a shallow body of water separated from the ocean by sandbars, coral reefs, or islands.
"The tourists swam in the calm waters of the tropical lagoon near their hotel."
Usage: Do not confuse this term with "lake," as a lagoon must be connected to the ocean, even if partially blocked off. It is often used in formal or geographical contexts rather than casual conversation about small ponds.
The word "lagoon" entered English through the French lagune and Italian laguna, ultimately deriving from the Latin lacuna. While it shares a distant ancestor with Old English words for sea, its specific meaning refers to a shallow body of water separated from the ocean by reefs or barrier islands.