What is an iterator in Python?

An iterator is an object that implements the iterator protocol, which consists of the methods __iter__() and __next__(). The __iter__() method returns the iterator object itself, and __next__() returns the next value from the underlying collection. When the data is exhausted, __next__() raises StopIteration. Iterators allow you to traverse containers lazily, consuming memory only as needed. Example: for x in [1, 2, 3]: print(x) internally creates an iterator over the list and repeatedly calls __next__() to get each element. Complexity is O(n) for iterating over n items, with O(1) additional space.