"Phantomnation" is one of the most famous examples of a "ghost word," or a word not found anywhere except in dictionaries. No longer recognized as a word, the term purportedly meant "illusion."
Most ghost words, including famous examples such as abacot, dord, and foupe, originally derived from printing errors. However, this was not the case with phantomnation.
The word first appeared in an 1820 dictionary by a man named Richard Paul Jodrell, entitled Philology on the English Language. Jodrell's work was very minor, but was sometimes used by other lexicographers as a supplementary reference. At a time when English spelling was still being standardized, Jodrell adhered to the eccentric view that any time a definition consisted of two words, the two words should be spelled as a single word. Thus his dictionary included entries for a large number of odd compounds, such as battlepainting, camelswallower, courtparasite, fellowcandidate, islandempress, latelypurchased, marriagesettlement, procurationmoney, pulpitsophistry, restlessrolling, stagegesture, tapestryhanging, and many others just as crazy.
"Phantomnation" was one of these words. Jodrell had found the term in a poem by the famous poet Alexander Pope, where Pope had used the two words "phantom nation" to describe a group of ghosts. Jodrell's definition read: "Phantomnation, n. A multitude of spectres. These solemn vows and holy offerings paid To all the phantomnations of the dead. Pope, Odyssey, b. x., v.627."
Most of these words were rightly ignored by other lexicographers, but for some reason phantomnation was pulled out of this list and treated as an actual word, on the mistaken assumption that it was formed using the word "phantom" and the suffix "-ation", with the letter "n" added for euphony. Then ignoring Jodrell's definition of "a multitude of spectres," which clearly indicated that this was just another of his word-smashings, they concocted a different definition out of whole cloth to fit with the the "-ation" theory.
Compiling a dictionary is a monumental task, and in an era when this was still being done in many cases by single individuals, it was often easier just to pirate definitions from other dictionaries, without bothering to go back and check original sources. Within a few years of Jodrell's work being published, the word phantomnation began appearing in other dictionaries, including John Ogilvie's Imperial Dictionary, which billed itself as an "expanded version" of Noah Webster's 1841 dictionary, and Worcester's Dictionary (produced by Webster's archrival in the dictionary trade, Joseph Emerson Worcester). In both of these cases, the term was defined as "illusion."
Finally in 1864, the term made it into Webster's dictionary itself, defined as ""Phantomnation, n. Appearance as of a phantom; illusion. Obs. and rare. Pope."
By around 1900, the word had been exposed as a fraud. By this time however, the word had appeared in literally dozens of dictionaries, and thus continued to haunt the world of lexicography, raising the interesting question of whether, if enough dictionaries contain a word, it can become a word even if it was not a word originally.
As late as 1934, the second edition of Merriam-Webster's gargantuan New International Dictionary (affectionately known as NI2) famously opted to include the word, but defined it only as "a ghost word combining the words phantom and nation - erroneously defined as a formation with the suffix -ation."
By becoming a ghost, the word "phantomnation" was now itself a kind of a phantomnation, having been transformed into an autonym.