Etymology and usage
Rāja (Sanskrit: राज) means "chief, best of its kind" or "king". Rāja yoga thus refers to "chief, best of yoga".
The historical use of the term Rāja yoga is found in other contexts, quite different than its modern usage. In ancient and medieval Sanskrit texts, it meant the highest state of yoga practice (one reaching samadhi). Hatha Yoga Pradipika, for example, refers to Hathayoga as one of the ways to achieve Rāja yoga.
The first known use of the phrase "Rāja yoga" occurs in a 16th-century commentary on a specific step in the Yoga Sūtras of Patañjali. The Hindu scholar Dattatreya, in his medieval era Tantric work named "Yogaśāstra", explains in 334 shlokas, principles of four yoga: Mantra yoga, Hatha yoga, Laya yoga and Raja yoga. Alain Daniélou states that Rāja yoga was, in the historic literature of Hinduism, one of five known methods of yoga, with the other four being Hatha yoga, Mantra yoga, Laya yoga and Shiva yoga. Daniélou translates it as "Royal way to reintegration of Self with Universal Self (Brahman)".
The term became a modern retronym when in the 19th-century Swami Vivekananda equated raja yoga with the Yoga Sūtras of Patañjali. This sense of meaning is different from Hatha Yoga Pradīpikā, a text of the Natha sampradaya, where it is a different practice.
The Brahma Kumaris, a new religious movement, teaches a form of meditation it calls "Raja yoga" that has nothing to do with either the precepts of Hatha Yoga or Patañjali's Yoga Sūtras.
Modern interpretations and literature that discusses Raja yoga often credit Patañjali's Yogasūtras as its textual source, but many neither adopt the teachings nor the philosophical foundations of the Yoga school of Hinduism. This mixing of concepts has led to confusion in understanding historical and modern Indian literature on Yoga.