A closed set is a concept in mathematics, particularly in topology. A set is considered closed if it contains all its limit points. This means that if a sequence of points within the set approaches a certain point, that point must also be included in the set. For example, the set of all real numbers between 0 and 1, including 0 and 1, is a closed set. In contrast, the set of all real numbers between 0 and 1, excluding 1, is not closed because it does not contain the limit point 1.