From All About Hinduism, Swami Sivananda Saraswati

No language is perfect. There is no proper equivalent word in English for the Sanskrit term dharma. It is very difficult to define dharma.

Dharma is generally defined as 'righteousness' or 'duty.' Dharma is the principle of righteousness. It is the principle of holiness. It is also the principle of unity. Bhishma says in his instructions to Yudhishthira that whatever creates conflict is adharma, and whatever puts an end to conflict and brings about unity and harmony is dharma. Anything that helps to unite all and develop pure divine love and universal brother-hood is dharma. Anything that creates discord, split and disharmony and foments hatred, is adharma. Dharma is the cementer and sustainer of social life.

The rules of dharma have been laid down for regulating the worldly affairs of men. Dharma brings as its consequence happiness, both in this world and in the next. Dharma is the means of preserving one's self. If you transgress it, it will kill you. If you protect it, it will protect you. It is your sole companion after death. It is the sole refuge of humanity.

Trying to define

That which elevates one is dharma. This is another definition. Dharma is that which leads you to the path of perfection and glory. Dharma is that which helps you to have direct communion with the Lord. Dharma is that which makes you divine. Dharma is the ascending stairway unto God.

Self-realization is the highest dharma. Dharma is the heart of Hindu ethics. God is the centre of dharma. Dharma means achara or the regulation of daily life. Achara is the supreme dharma. It is the basis of tapas or austerity. It leads to wealth, beauty, longevity and continuity of lineage. Evil conduct and immorality will lead to ill fame, sorrow, disease and premature death. Dharma has its root in morality and the controller of dharma is God Himself.

Maharshi Jaimini defines dharma as that which is enjoined by the Vedas and is not ultimately productive of suffering. Rishi Kanada, founder of the Vaiseshika system of philosophy, has given the best definition of dharma, in his Vaiseshika Sutras:

Yato bhyudayanihsreyasa siddhih sa dharmah.

That which leads to the attainment of abhyudaya (prosperity in this world) and nihsreyasa (total cessation of pain and attainment of eternal bliss here after) is dharma.

The four Vedas, the smriti texts, the behaviour of those who have entered into their spirit and act according to their in junctions, the conduct of holy men and the satisfaction of one's own self - these are the bases of dharma, according to Manu.

In the matter of dharma, the Vedas are the ultimate authority. You cannot know the truth about dharma through any source of knowledge other than the Vedas. Reason cannot be the authority in the matter of dharma. Among the scriptures of the world, the Vedas are the oldest. This is supported by all leading scholars and antiquarians of the entire civilized world. They all declare with one voice, that of all books so far written in any human language, the Rig Veda Samhita is undoubtedly the oldest. No antiquarian has been able to fix the date when the Rig Veda Samhita was composed or came to light.

The changing dharma

Just as a doctor prescribes different medicines for different people according to their constitution and the nature of their disease, so also Hinduism prescribes different duties for different people. Rules for women are different from the rules for men. The rules for different varnas and ashramas vary. However, non-violence, truth, non-stealing, cleanliness and control of the senses, are the duties common to all men.

Dharma depends upon time, circumstances, age, degree of evolution and the community to which one belongs. The dharma of this century is different from that of the tenth century.

There are conditions under which dharma may change its usual course. Aapad dharma is such a deviation from the usual practice. This is allowed only in times of extreme distress or calamity.

What is dharma in one set of circumstances becomes adharma in another set of circumstances. That is the reason why it is said that the secret of dharma is extremely profound and subtle. Lord Krishna says in the Bhagavad Gita (16:24):

Tasmaachchhaastram pramaanam te kaaryaakaaryavyavasthitau; Jnaatvaa shaastravidhaanoktam karma kartumihaarhasi.

Let the scriptures be the authority in determining what ought to be done and ought not to be done. Knowing these rules and regulations, one should act here in this world accordingly and be elevated gradually.