A bidirectional iterator is a type of iterator that allows traversal of a data structure in both forward and backward directions. This means you can move to the next element as well as return to the previous one. Bidirectional iterators are commonly used in data structures like doubly linked lists and deques, where accessing elements in both directions is beneficial. In programming, bidirectional iterators are often implemented in languages like C++ and Python. They provide a standard interface for iterating through collections, making it easier to manipulate data. This flexibility enhances the usability of algorithms that require access to elements in both directions.