prune the French word for plum, means in English a dried plum. The word has been used in English in this sense since medieval times (although for several centuries it could also, confusingly, mean a fresh plum).
Prunes all come from a group of oval, black-skinned plums. Their special characteristics are a very high level of sugar, which allows them to be sun dried without fermenting (although nowadays the process is often speeded by drying machinery) and a ‘free’ or easily detached stone, which is uncommon among plums. Prunes turn completely black in drying as the result of enzyme action. This would be considered unacceptable in any other fruit, but is deemed normal in prunes.