A true word used in Hinduism is Ananda, a state of causeless bliss and joy. There are many theories about True Happiness (sukha). Happiness is a state of mind and is achieved in many different forms for different people. Some may say happiness is fulfilment of desires, e.g. people striving for something in their lives and upon achieving their goal, they can say they found happiness. But, True Happiness, according to the sages, is a State of Being that cannot be acquired from the world outside, but comes from within. When one is at peace within oneself, then there is true joy and calmness.
When you are in a state of ananda, you are able to find joy in little things like being outside under blue skies, a bird singing, or being in sunshine or the rain. When you are in a state of ananda, you find a purpose in life by doing greater good for humanity.
The other side of happiness is dukha (sadness) that comes out of not accepting the world as it is but desiring a world of your dreams and then having an attachment to the desire. Desires and attachments to worldly things bring miseries in life.
Other reading material:
Relevant videos: Swami Nikhilanand Hinduism Q & A: What is True Happiness
Contributor: Mukta Vadera