[Oni-Paranoia]

Happiness - A moment in when no negative or neutral emotions exist as a majority. (This isn't my exact definition but it's something similar.)

By the way, the best definition is one that can apply to all situations and be true at the same time. (Some people tend to forget about that).

Also, the happier we are, the easier it is to make us unhappy. Just stating.

EDIT:

Originally Posted by Hakulyte

We could complicate the question a little more by saying something along the lines of do you need to experience true sadness to find true happiness?

The more negative emotions, such as sad, you experience, the easier it is to be happy. There is no such thing as "true sadness or happiness" IMO. Most emotional experiences are based on personal experiences.