I happened to be discussing with Shivananda(links to homepage, not blog) about an eternal question - 'What is happiness?'. The question is probably one of the most asked and most of the answers given are all valid. Here are some set of answers Shivananda came out with. He picked up the definitions from I don't know where.
Happiness is being in perfect harmony with ourselves.
Happiness is feeling good about ourselves.
Happiness is feeling a sense of satisfaction when you can help someone.
Happiness is being in love with our inner self.
But above all, happiness is when we forget ourselves doing what we enjoy the most!
If you have a definition of yours, post it here. And here is the definition that I like the most. Can't recall where I read it first - 'Happiness is the sense of feeling when you are contempt with the current state of being and not wanting to go anywhere from here'. What would I do when I ever feel that transendental feeling of complete happiness? I would just sit back wherever I am, elated, not knowing what to do.... It is difficult to say with words....
In a different thread, I recalled this famous statement with which, Tolstoy's novel 'Anna Karenina' begins. All happy families resemble one another; every unhappy family is unhappy in its own way
Update(4th Feb): There was an ugly typo!! Above, in my definition of happiness, 'contempt' was to be written as 'content'. That was a bad one. Thanks to 'ahem' for pointing it out.
Nidhi posted her definition of happiness: 'Happiness is doing what you like!'. I liked it, simple but very true.