To compose two functions means to call one on the result of the other.
Related concepts: Function
Closest Wikipedia entry: Function composition (computer science) — In computer science, function composition is an act or mechanism to combine simple functions to build more complicated ones. Like the usual composition of functions in mathematics, the result of each function is passed as the argument of the next, and the result of the last one is the result of the whole. Programmers frequently apply functions to results of other functions, and almost all programming languages allow it.