What is a 'ghost word'?

The Second Edition of the 20-volume Complete Oxford English Dictionary contains full entries for 171,476 words in current use. An estimated 240 of  these are ghost words.

A ghost word has never existed outside dictionaries. They are published by mistake but can have a long life if the error is not spotted.

The term is usually attributed to the philologist  and lexicographer Walter Skeat. In an address to the Philological Society in 1886, he described how a new word morse was erroneously identified in Walter Scott's popular novel The Monastery (1820).

“… dost thou so soon morse thoughts of slaughter?” Two correspondents proposed their own etymologies for morse. One proposed that it meant “to prime (as with a musket),” from Old French...The other correspondent proposed that morse meant “to bite” (from Latin morsus) source

It was in fact a misprint of the word nurse