a very large animal which is supposed to live in Loch Ness, a large very deep lake in northern Scotland. Although some people say that they have seen the monster, its existence has never been proved, and for most people it is just a story. It is often shown in pictures as a creature like a long black or green dinosaur, and it is informally called Nessie.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.
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