As long as the language police have chimed in...
Originally Posted by Steve Smith
The word decimate literally means to kill one in ten, describing the practice of the Roman military, which would punish disobedient or substandard units by randomly selecting 10% of the men in the unit and putting them to death (presumably this motivated the other 90% to get with it...).
In its more modern and extended meaning, it can be defined as: killing large numbers of things (as in "The population of darkroom mice has been decimated in recent years due to their getting caught in light traps."), or it can mean large scale destruction or severe reduction ("Film manufacture has been decimated in the last decade." [just had to use the Latin "ten" twice...], or "Al Qaeda has been decimated." [Pres. Obama] ).
Note that "decimate" always has a negative implication, that of destruction or death; it is not simply a neutral 10% reduction. I'd never say "I've decimated my weight." if I lost 10% of my bulk... that would be positive, wouldn't it?
The above from Cambridge and Longmans.