In computer programming, a parameter, a.k.a. formal argument, is a variable that represents an argument, a.k.a. actual argument, a.k.a. actual parameter, to a function call. A function's signature defines its parameters. A call invocation involves evaluating each argument expression of a call and associating the result with the corresponding parameter.

For example, consider the Python function

Variables x and y are parameters, each of type int. For call add(2, 3), the expressions 2 and 3 are arguments. For call add(a+1, b+2), the arguments are a+1 and b+2.

Parameter passing is defined by a programming language. Evaluation strategy defines the semantics for how parameters can be declared and how arguments are passed to a function. Generally, with call by value, a parameter acts like a new, local variable initialized to the value of the argument. If the argument is a variable, the function cannot modify the argument state because the parameter is a copy. With call by reference, which requires the argument to be a variable, the parameter is an alias of the argument.

The following C source code defines a function named salesTax with one parameter named price; both typed double. For call salesTax(10.00), the argument 10.00 is passed to the function as the double value 10 and assigned to parameter variable price, and the function returns 0.5.

The terms parameter and argument may have different meanings in different programming languages. Sometimes they are used interchangeably, and the context is used to distinguish the meaning. The term parameter (sometimes called formal parameter) is often used to refer to the variable as found in the function declaration, while argument (sometimes called actual parameter) refers to the actual input supplied at a function call statement. For example, if one defines a function as def f(x): ..., then x is the parameter, and if it is called by a = ...; f(a) then a is the argument. A parameter is an (unbound) variable, while the argument can be a literal or variable or more complex expression involving literals and variables. In case of call by value, what is passed to the function is the value of the argument – for example, f(2) and a = 2; f(a) are equivalent calls – while in call by reference, with a variable as argument, what is passed is a reference to that variable - even though the syntax for the function call could stay the same. The specification for pass-by-reference or pass-by-value would be made in the function declaration and/or definition.

Parameters appear in procedure definitions; arguments appear in procedure calls. In the function definition f(x) = x*x the variable x is a parameter; in the function call f(2) the value 2 is the argument of the function. Loosely, a parameter is a type, and an argument is an instance.

A parameter is an intrinsic property of the procedure, included in its definition. For example, in many languages, a procedure to add two supplied integers together and calculate the sum would need two parameters, one for each integer. In general, a procedure may be defined with any number of parameters, or no parameters at all. If a procedure has parameters, the part of its definition that specifies the parameters is called its parameter list.