I commonly see the word "decimate" used quantitatively, to mean "to reduce by 90%".

So commonly, in fact, that I consider it correct.

Yes, yes. In Latin, it means "to reduce by 10%", which doesn't sound very scary unless you're a Centurion awaiting the drawing of lots.

That's Latin. In English, having a word for "knocked down by an order of magnitude" is useful. We have, fortunately, not retained decimation as a form of punishment. "to reduce by 90%" is also closer in feeling to the qualitative meaning, "to desolate emotionally".

I was prompted to write this when someone described traffic as "literally decimated" when occupancy of a highway reaches a certain number. It occurred to me that he or she was right to do so. Your useage may vary.