A hapax legomenon is a hapax legomenon with reference to a particular corpus. In this context, a "corpus" is a set of words, or more generally a set of works under consideration.

Any corpus of one word is, by construction, composed entirely of of hapax legomena. I think the wikipedia page is fairly clear on the subject, honestly. In general, they're a phenomenon which is fairly obvious and uninteresting.

Where it becomes slightly more interesting is when, in some long text, an author uses a word the no one knows, and doesn't bother to explain it, and never uses it again. It becomes particularly interesting when trying to translate important ancient texts... what the devil did this word really mean?