It is "unfriend" -- which means to remove someone as a "friend" on a social networking site. For instance people might say, "I decided to unfriend my roommate on Facebook after we had a fight."
A spokesperson says the word has both currency and potential longevity.
The runner-ups are almost as interesting. They include: "hashtag," "funemployed," "birther," "death panel," and "teabagger" which is someone who protested against the Obama tax and stimulus plans.
The New Oxford American Dictionary has come out with the word of the year.