Rāja Yoga ("royal yoga", "royal union", also known as Classical Yoga and Ashtanga Yoga) is concerned principally with the cultivation of the mind using meditation (dhyana) to further one's acquaintance with reality and finally achieve liberation.

Raja yoga was first described in the Yoga Sutras of Patanjali, and is part of the Samkhya tradition.

In the context of Hindu philosophy Raja Yoga is known simply as yoga. Yoga is one of the six orthodox (astika) schools of Hindu philosophy and forms an integral part of the spiritual practices of many Hindu traditions. The term is also used as the title of an entirely unrelated practice by the Brahma Kumaris and Prajapita Brahma Kumaris.

The term Rāja Yoga is a retronym, introduced in the 15th-century Hatha Yoga Pradipika to distinguish the school based on the Yoga Sutras of Patanjali from the more current school of Hatha Yoga expounded by Yogi Swatmarama. Currently (2011 C.E.)the term is also used to describe the meditation practice of the Brahma Kumaris.

Raja (also spelled rajah, from Sanskrit राज rāja-) is an Indian term for a monarch, or princely ruler of the Kshatriya varna. The female form, the word for "queen", mainly used for a raja's wife, is rani (sometimes spelled ranee), from Sanskrit राज्ञी rājñī, or ratu, dato, datuk, or datu in Southeast Asia. The title has a long history in the Indian subcontinent and Southeast Asia, being attested from the Rigveda, where a rājan- is a ruler, see for example the (dāśarājñá), the "battle of ten kings".

Sanskrit rājan- is cognate to Latin rēx (genitive rēgis), Gaulish rīx, Gaelic rí (genitive ríg), etc., originally denoting heads of petty kingdoms and city states. It is ultimately derived from a PIE *h₃rēǵs, a vrddhi formation to the root *h₃reǵ- "to straighten, to order, to rule". The Sanskrit n-stem is secondary in the male title, apparently adapted from the female counterpart rājñī which also has an -n- suffix in related languages, compare Old Irish rígain and Latin regina. Cognates of the word Raja in other Indo-European languages include English reign and German reich.

Yoga (Sanskrit, Pāli: योग, yoga) is the Hindu practice of physical, mental, and spiritual discipline, originating in ancient India. The goal of yoga, or of the person practicing yoga, is the attainment of a state of perfect spiritual insight and tranquility while meditating on the Hindu concept of divinity or Brahman. The word is associated with meditative practices in Hinduism, Jainism, and Buddhism.

Within Hindu philosophy, the word yoga is used to refer to one of the six orthodox (āstika) schools of Hindu philosophy. Yoga in this sense is based on the Yoga Sutras of Patanjali, and is also known as Rāja Yoga to distinguish it from later schools. Patanjali's system is discussed and elaborated upon in many classical Hindu texts, and has also been influential in Buddhism and Jainism. The Bhagavad Gita introduces distinctions such as Jnana Yoga ("yoga based on knowledge") vs. Karma Yoga ("yoga based on action").

Other systems of philosophy introduced in Hinduism during the medieval period are bhakti yoga, and hatha yoga.