Ghost words are actually and in fact words that entered the dictionary by mistake and were either discovered and deleted—ghosted, as it were—or gathered lovingly into the language in spite of their ghostly ancestry.
"Dord" is an example of a word taken into, then out of, the dictionary. It was written down as a synonym for "density" because in the original entry, the lexicographer added, helpfully, after "density," "D or d," meaning that "density," a term of physics, could be spelled with or without a capital D: "Density, D or d"—"Density, Dord." It was an easy mistake to make, but it became a ghost word when it was removed from the dictionary in a huff.
"Syllabus" is an example of a word with a ghostly, i.e., nonexistent, ancestry that was left in the dictionary in spite of the error that birthed it. In some 17th-century scriptorium, a scribe copying words into a dictionary came to the Latin word sittybas, defined as a parchment label. In the dim light in which he was working, he apparently didn't notice the crosses on the "t"s and created the word "syllabus." In time, the meaning also got twisted, from a "parchment label" to "a list," and on to the most common modern meaning, "a list of lessons for a class."
But I like to think of ghost words as wraiths, hovering into the language and then evaporating. "Crapulous," for instance, a word for the way you feel after you've eaten too much—isn't it a shame that that word became a ghost? Elflock— hair that elves have tangled—is another ghost word (by my definition) we could use today—much better, don't you think, than "bad hair day"?
I wish curglaff, the shock felt at a cold-water plunge, hadn't turned into a ghost. I would love to be able to say, as I plunged into a lake at 10,000 feet, "Wow! That was a great curglaff!" And I still could, I guess, even if no on knew what I meant, except that I probably won't remember it. It has only a spectral existence, flitting through the linguistic atmosphere as insubstantial and unsnatchable as a ghost.