Russell's paradox is a fundamental problem in set theory, discovered by the philosopher Bertrand Russell in 1901. It arises when considering the set of all sets that do not contain themselves. If such a set exists, it leads to a contradiction: if it contains itself, then by definition it should not; if it does not contain itself, then it must contain itself. This paradox highlights inconsistencies in naive set theory, prompting mathematicians to refine the foundations of set theory. As a result, more rigorous frameworks, such as Zermelo-Fraenkel set theory, were developed to avoid such contradictions and ensure logical consistency in mathematics.