The term sanatana refers to that which is eternal and dharma is generally understood to be law or the order of things. Thus, the term Sanatana Dharma as it began to be articulated in the 19th century referred to an eternal law. When we look at the history of the term, the term dharman is known as early as in the Rig Veda, but it is primarily in terms of the upholding of the cosmic order, or rita that it appears; sanatana does not appear in the Vedic samhita.
janataweekly.org/sanatana-dharmas-historical-roots-reveal-a-complex-journey/