The only truly dead language is one which can't be understood anymore. All other languages are in various states of evolution.

There are various states to evolution, of course. Latin, and other languages people consider to be dead, such as Ancient Greek, have reached inertia. Any language, once its rules (grammar, spelling, etc.) are written down, will reach inertia. Any change in the language formed afterwards becomes a new form of the language that continues to evolve until it, too, reaches inertia.

Case in point: Victorian English.