1. Collectively, all words with which one is familiar but is unable to define.
2. Words that one would commonly be expected to know based on his or her education or socio-economic class but does not.
3. Particular words, generally of an erudite nature, which one finds perpetually befuddling despite previous attempts to ascertain their meaning.
"Four years of college and I all I have to show for it is a massively inflated perplexicon!"