Tagdef has the list of definition of hash tags used in twitter. You don't like the definition of hash tag present in tagdef? Don't worry you ca define your own full form or definition. I think its time to explore tagdef.
I hope you know twitter. Its social networking and micro blogging site. Its enough explore your self.
Hey I almost forgot, Vivek K is my twitter account :-)
PS:#tcot is most used hashtag in twitter