The concept of “dharma” is difficult to convey in English. The term is an ancient one equivalent to the Persian word “daena” which means something like insight and revelation. In Zoroastrianism, Daena has been explained as a journey that enables the soul to see light at the end of life.
In the Upanishads, dharma is defined as truth, but both dharma and truth are philosophies unto themselves. They are short words that encompass entire belief systems. Dharma is a lodestar, an abiding principle, not only for Hinduism, but also for her sister religions, Buddhism, Sikhism and Jainism. These faiths have arisen from different aspects of the Hindu world view and have evolved with their own specificities and emphases.
The word Hindu describes the original inhabitants of the Indus River Valley, but today many followers of Hinduism prefer to describe their faith as the Sanatana Dharma, an abiding principle which means The Eternal Order or Way.
See Chapter 8 of On Hinduism by Irina Gajjar to appreciate the full meaning of “dharma” in Eastern religions.