In mathematics, specifically category theory, a modification is an arrow between natural transformations. It is a 3-cell in the 3-category of 2-cells (where the 2-cells are natural transformations, the 1-cells are functors, and the 0-cells are categories). The notion is due to Bénabou.
Given two natural transformations , there exists a modification such that:
The following commutative diagram shows an example of a modification and its inner workings.