The term denotes the "eternal" or absolute set of duties or religiously ordained practices incumbent upon all Hindus, regardless of class, caste, or sect.
[1] Many Hindus in the Indian subcontinent call themselves Sanatanis, that is, those who follow the 'eternal dharma', to evoke a certain homogeneity in Hinduism, although it's also sometimes used by Jains and Buddhists who believe in concepts like rebirth.
The word comes from the Sanskrit root "dhṛ" (धृ) which means "to sustain" or "that which is integral to something" (e.g., dharma of sugar is to be sweet, fire to be hot).
A person's dharma consists of duties that sustain them according to their innate characteristics which are both spiritual and material, generating two corresponding types:[8] According to the notion of sanatana-dharma, the eternal and intrinsic inclination of the living entity (atman) is to perform seva (service).
[2] In current-day usage, the term sanatana dharma is diminished and used to emphasize a "traditional” or sanatani ("eternalist") outlook in contrast to the socio-political Hinduism embraced by movements such as the Arya Samaj.