An argument is the actual value assigned to a parameter when a subroutine is called.

When discussing code that is calling into a subroutine, any values passed into the subroutine are the arguments, and the place in the code where these values are declared is the parameter list. When discussing the code inside the subroutine definition, the variables in the subroutine's parameter list are the parameters, and the values of the parameters while the subroutine is processing are the arguments.