A function is a program written to optionally accept input parameters, perform an operation, or return a single value. A function returns only one value per execution.
Three important components form the basis of defining a function. The first is the input parameter list. It specifies zero or more arguments that may be passed to a function as input for processing. These arguments or parameters may be of differing data types, and some are mandatory while others may be optional. The second component is the data type of its resultant value. Upon execution, only one value is returned by the function. The third encapsulates the details of the processing performed by the function and contains the program code that optionally manipulates the input parameters, performs calculations and operations, and generates a return value.