An element of B that corresponds to some element x of A

Let ƒ be a function from set A to set B (ƒ: A → B), then

An element of B that corresponds to some element x of A is denoted by ƒ(x), and it is called the of x under ƒ.

Reference

Introduction to Differential Calculus (Systematic Studies with Engineering Applications for Beginners) - 2012