As any pedant will tell you, decimate means “to destroy a tenth of something.” Of course, its modern usage has been expanded to this:
to destroy a large number of (plants, animals, people, etc.)
to damage or destroy a large part of (something)
Almost every modern use of the word decimate is in this expanded sense. However, Merriam-Webster, Dictionary.com, The Free Dictionary, and Oxford Dictionaries all list the historical “destroy a tenth” definition along with the modern definition, and it’s not uncommon for people to argue that this is the only definition that should be allowed for the word--even though the word in that sense is rarely, if ever, used (The British National Corpus gives 23 results for decimate, 21 of them using the modern definition and the remaining two complaining about the use of the modern definition).
However, there are plenty of other words whose definitions have been expanded beyond their original number-specific etymologies, such as alternative and combine, and you don’t see anyone causing a stink about those. Is there a reason why people want to cling so desperately to decimate’s literal definition? What’s the point of trying to enforce a definition that no longer has any de facto existence?
EDIT: Some people asked for examples of people complaining about decimate being used with the expanded definition rather than the historical one. Here are some examples:
This website lists using decimate in this way as an error.
This Oxford Dictionaries blog addresses the "linguistic pet peeve" of people who complain about decimate being used this way.
This NPR article quotes the senior supervising producer of their Arts & Life section, who says,
"I think the people who defend the original meaning of 'decimate' do so in part because we feel it's sad to lose a word for describing something so precisely and in a way that evokes such history. I guess one can substitute 'collective punishment' or 'culling.' But every time we let word meanings bleed together, it's like a little star winking out."