Lexicographers call them “ghost words” -- essentially, words that appear in reference works because of an error. The most famous ghost word is probably “dord,” which found its way into the second edition of Webster’s New International Dictionary in 1934. In that dictionary, "dord" is defined as a synonym for density used by physicists and chemists. The entry apparently escaped the attention of proofreaders and remained in the dictionary until 1939, when a sharp-eyed editor flagged it as fake -- yet the change wasn’t actually made until 1947. "Dord" seems to have slipped into the dictionary when the phrase "D or d" (abbreviations for density) was accidentally re-written as a single word, and "dord" was born.
Today, a lot of newly-coined words (neologisms) created on the Internet find legitimacy in dictionaries, such as meme, NSFW, and jeggings.
There's a word for that: