Arguments and parameters refer to the variables or expressions passed from the caller of a function (or invoker of a macro) into the function (or macro).

The ISO C Standard supports this distinction:

• argument: that which is passed into a function by its caller (or macro by its invoker)
• parameter: that which is received by the function or macro

although the qualifiers "actual" and "formal" in deprecated usage can change this.

Definitions from the Standard

The C99 Standard defines an argument as an

expression in the comma-separated list bounded by the parentheses in a function call expression, or a sequence of preprocessing tokens in the comma-separated list bounded by the parentheses in a function-like macro invocation

and notes the synonyms "actual parameter" (deprecated) and "actual argument".

It defines a parameter as an

object declared as part of a function declaration or definition that acquires a value on entry to the function, or an identifier from the comma-separated list bounded by the parentheses immediately following the macro name in a function-like macro definition

and notes the synonyms "formal argument" (deprecated) and "formal parameter".