A bijection is a special type of function in mathematics that establishes a one-to-one correspondence between two sets. This means that every element in the first set is paired with exactly one unique element in the second set, and vice versa. In simpler terms, no element is left unpaired in either set, making the function both injective (one-to-one) and surjective (onto). Bijections are important in various fields, including set theory and combinatorics, as they help in understanding the size and structure of sets. When two sets can be bijectively mapped, they are considered to have the same cardinality, or size, which is a fundamental concept in mathematics.