Here is Bhisma’s definition in the Mahabharata.
Not harming, truthfulness, remaining free from anger, and charity, are the four practices you must adhere to, Ajatasatru. This is the Sanatana Dharma.
In this verse, we have another delineation of Sanatana Dharma, this time given to King Yudhishthira by Bhishma, his sagacious grandfather, just before the latter decides to depart from this world. The definition given here is rather similar to that offered above by Savitri, including ahimsa , remaining free of anger, and giving gifts in charity. To these virtues, Bhishma also adds truthfulness, which can probably be taken as meaning honesty in all one’s dealings. These are of course virtues that should be adhered to by all human beings, but they are also indicators of spiritual achievement. Spirituality is not some vague notion, or acts of obscure religiosity, but is very practical. The state of consciousness attained can best be measured by the extent to which one is willing and able to avoid harming and anger, to give frequently to those in need, and to be honest in all one‟s dealings. This is why the Sanatana Dharma is explained in this very practical way.