The word Dharma in Hinduism is the most confusing. If you check any of the authentic Sanskrit dictionaries you shall find that the meaning of the word Dharma will have many pages !! Here is the explanation of Sanatana Dharma.
Everything in the world, the Einsteinian world of objects, the objects of the world outside must have properties. Sugar – it must have its property, Fire – it must have its property. The properties of a thing fall under 2 categories – essential property and non-essential property. This essential property of a thing because of which the thing is the thing, in the absence of which, how much it may look like the thing, it is not the thing. This essential property is called its Dharma. Thus what is your Dharma? The Dharma of all living beings is the Self – the life in you, because of which you are what you are.
Once the life is gone, you look, within a 48 hours you look like an eggplant. Worms will come out of it. So long as you are alive, you are efficient physically, dynamic mentally, and highly intellectual – a genius !! You are serving the society. OK. When? So long as life is there. But once it is gone you are no more, you are wiped out, rubbed out. THINK. So the common Dharma of all living beings – plant, animal and human is the supreme Dharma. (Reality, GOD) That is why this is called as Sanatana Dharma –
This part of Existence in you, this spring of Consciousness in us, this touch of Life in all living beings, let us call it as OM.