Mistake

In 1934, Webster's second edition printed 'dord', a Word that never existed. Defined as meaning density, it began as an editor's slip 'D or d' (D or d abbreviates density), misread as one word. The ghost entry sat in the Dictionary five years before anyone caught it: a 'ghost word'.

Google's name was created by accident. The founders wanted to use "Googol," meaning 10 to the 100th power, but an investor misspelled it on a check — and "Google" stuck.