Someone whose social behavior is extremely abnormal. Sociopaths are interested only in their personal needs and desires, without concern for the effects of their behavior on others. "sociopath." The American Heritage® New Dictionary of Cultural Literacy, Third Edition. Houghton Mifflin Company, 2005. 01 Jun. 2007. dictionary.com http://dictionary.reference.com/browse/sociopath.
(Yes it's an american dictionary but I can't find my Oxford one so it'll have to do)
So basically, to be a sociopath you have to be selfish and unusual. But it's more than that, I would draw a subtle line between sociopathy and my hard-line libertarianism. I do act, in general, in my own interest but it's not because I don't care about anyone else, it's because I think it's the best way for everyone to prosper. I believe that, if society is left to it's own devices and everyone works, to a great extent, only for their own good, then work becomes valuable enough that everyone can succeed by their own labour. So no, I'm not a sociopath, just a little unusual, phew :)
Disagree? Let me know.