Sanatan translates to eternal. Dharma is universal order and conformity to natural righteousness.
Saints and seers of India codified their experienced spirituality into the ethics, arts, sciences, architecture, food, practices, and rituals based on the observation of the order in the universe around them – something that has always existed and is eternal. Hence the term Sanatan Dharma, which is too broad to be any kind of an “ism”.
Hinduism is a modern term invented after ”Indus” valley civilization which compromises the essence of Sanatan Dharma.
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Other reading material: Short answers to real questions about Hinduism, Rajeev Singh: About Sanatan Dharma
Relevant videos: Swami Nikhil Anand Hinduism Q & A: What is the Original Name Of Hinduism, What is Sanatan Dharma?,
Contributor: Geetha Ravula