[1] The typically indicates that for the definition to hold, only a "critical mass" of the descriptors must be filled. Something that fills all other criteria, but does not occur near adolescent girls, for example, could still be well thought of as a poltergeist. This is similar to the cluster theory of proper naming advocated by Wittgenstein (but not so loose as that put forward by Searle).
this can only be what Lewis means what he says that he does not