Function composition (often denoted as fg or f(g(x))) is a mathematical operation that takes two functions f and g and produces a new function h such that h(x) = f(g(x)). In other words, the output of g is used as the input to f.
Function composition is an important operation in mathematics, and it has many applications in a variety of fields, including computer science, physics, and engineering. For example, function composition can be used to model the behavior of complex systems, such as the flow of water through a pipe or the motion of a projectile.