Hashtag, the word used ubiquitously on Twitter, has been voted the Word of 2012 by the American Dialect Society. The criteria the Society uses when deciding on a winner is that the word (or phrase) needs to have been 'newly prominent or notable' in the past year.
Runners-up included another internet-related item, YOLO, an acronym for 'you only live once', and words which were on other organisations' Words of the Year lists, such as fiscal cliff and Gangnam style.
There are various categories in the American Dialect Society's annual competition. Beardruff (dandruff on one's beard) appears on the 'Most Useful' list, mansplaining (a man's condescending explanation to a female audience) is on the 'Most Creative' list, Dunlop effect (when the stomach protrudes over ill-fitting pants) is among the 'Most Outrageous' words, and one of the words voted 'Least Likely to Succeed' is phablet (a device of the size between a smartphone and a tablet).
Here is the page on the American Dialect Society's website.