“Un-” is a negative prefix denoting the opposite of the word to which it is affixed. The nearest equivalent prefix is “non-”, which denotes the same thing. “Un-”, however, also possesses a connotation (in many applications of the prefix) of time, where previously the state or thing that is not, once was. In the case of zombies, the term “undead” is used to refer to them as a collective whole, as a race or species.