For years, I have noted this word being confused with "devastated". In it's strictest sense, the word means the loss of 1/10th of a total, a very precise proportion. The word is derived from Latin. Classic examples of it's use are when Roman officers would kill one man in ten, to punish a unit for cowardice or mutiny.
It is perfectly reasonable to use the term in a less precise sense. For example, if the owner of an apple orchard were to claim that her trees had been "decimated" in a severe windstorm, no one in his right mind would find it necessary to check on this to see if precisely 500 trees had blown down from a total of 5,000.
Last week, watching the PBS News hour, I heard one very sharp commentator state that some entity had been "devastated and decimated". That was quite absurd! It is akin to say that the weather was "freezing and a bit chilly".