An input iterator is a type of iterator in programming that allows you to read data from a collection, such as an array or a list, one element at a time. It provides a way to traverse through the elements without modifying them. Input iterators are commonly used in algorithms that require reading data, making them essential for efficient data processing. These iterators typically support operations like incrementing to the next element and dereferencing to access the current element's value. They are a fundamental part of the C++ Standard Template Library (STL), enabling developers to work with various data structures in a consistent manner.