A functor is a concept from category theory in mathematics and computer science that describes a mapping between categories. It allows for the transformation of objects and morphisms (arrows) from one category to another while preserving their structure. This means that if you have a function that operates on data, a functor can apply that function to all elements within a data structure, like a list or a tree. In programming, particularly in languages like Haskell and Scala, functors are often used to handle data in a consistent way. They provide a way to apply functions to wrapped values, such as those found in Option or List types, without needing to unwrap them first. This abstraction helps manage complexity and enhances code reusability.