Defining happiness could be one of the most difficult task, it can differ from person to person depending on the situation each individual are facing in their day-to-day life. Everybody has a different perspective to the term 'Life' . Some people 'live their life' and some people 'try to survive in their life.' So their views wont be the same when you are asking them, how happiness can be defined.
In short what I have found is, being acceptable to the current situation (situation which comes your way or rather thrown at you) is the best way to be happy.