A function is a set of instructions that when executed accomplishes something. The function main executes automatically when you run a program. Other functions execute only when they are activatedthat is, called. Predefined functions are available in C++ libraries called header files. A particular header file may contain several functions.

One example is the pow function pow(x,y) = xy. The numbers x and y are called the arguments or parameters of the function pow.

An expression such as pow(2,3) is called a function call, which causes the code attached to the function pow to execute. The header file cmath contains the specification of the function pow.