A fossil word is a word that has fallen out of common usage except in specific idioms or sayings. This generally only refers to words that were once in popular usage, but no longer are -- for example, today we are likely to encounter 'fettle' in the phrase 'in fine fettle', but it was once a common word used in much the same way as we now use 'condition'.

Proper names are generally not considered fossil words -- Molotov cocktail is certainly the only use of the word Molotov that most people can give off hand, but it is more likely to be considered an eponymous term than a fossil. Foreign words are also not generally considered fossil words, for obvious reasons. Words such as 'ami' and 'vici' may become unintelligible to hoi polloi when used in isolation, but in this case the words were never in popular usage in our language, and so cannot be fossilized. When a fossil word has been completely absorbed into a compound word, it is called a cranberry morpheme, after the 'cran' in 'cranberry.

Examples include, but are certainly not limited to: