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(Or "arg") A value or reference passed to a function, procedure, subroutine, command or program, by the caller. For example, in the function definition
square(x) = x * x
x is the formal argument or "parameter", and in the call
y = square(3+4)
3+4 is the actual argument. This will execute the function square with x having the value 7 and return the result 49.
There are many different conventions for passing arguments to functions and procedures including call-by-value, call-by-name, call-by-reference, call-by-need. These affect whether the value of the argument is computed by the caller or the callee (the function) and whether the callee can modify the value of the argument as seen by the caller (if it is a variable).
Arguments to functions are usually, following mathematical notation, written in parentheses after the function name, separated by commas (but see curried function). Arguments to a program are usually given after the command name, separated by spaces, e.g.:
cat myfile yourfile hisfile
Here "cat" is the command and "myfile", "yourfile", and "hisfile" are the arguments.