a river in northeast France, or the area close to this river where several important battles were fought during World War I. The biggest of these began in July 1916, when 60,000 British soldiers were killed or injured on the first day of the battle. After five months of fighting, the British army had moved forward by just a few miles, and had lost hundreds of thousands of men. Because of this, the Somme is connected in people's minds with the terrible waste of life in World War I.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.