I rather like Thoreau's definition:
"Happiness is like a butterfly; the more you chase it, the more it will elude you, but if you turn your attention to other things, it will come and sit softly on your shoulder"
It is certainly the one that most chimes with my personal experience.
(and btw the same applies to Robins in the garden)
Although it also fits quite happily with Aristotle's notion:
"Happiness depends upon ourselves"
An interesting read, thanks NS