Function, in mathematics, an expression, rule, or law that defines a relationship between one variable (the independent variable) and another variable (the dependent variable).
The concept of a function was formalized at the end of the 19th century in terms of set theory, and this greatly increased the possible applications of the concept. A function is often denoted by a letter such as f, g or h.
A function in maths is a special rule or relationship that assigns each input (usually called x) exactly one output (usually called y or f (x)). In other words, for every value you put into the function, you get only one value out.