Raja Yoga ("royal yoga", "royal union", also known as Classical 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.
The term
The term Raja 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. The term was later used to describe the meditation practice of the Brahma Kumaris.
Raja Yoga is sometimes referred to as A??anga (eight-limbed) yoga because there are eight aspects to the path to which one must attend. Patanjali himself called his system of yoga 'Kriya Yoga' as is known in his first sutra of the second chapter: Tapas svadyaya ishvarapranidhanani kriya yogah (2:1), "Discipline, insight, and devotion are the pillars of Kriya Yoga". This is not to be confused with the Ashtanga Vinyasa Yoga of K. Pattabhi Jois.
Concept
Raja Yoga is so-called because it is primarily concerned with the mind. The mind is traditionally conceived as the "king" of the psycho-physical structure which does its bidding (whether or not one has realized this). Because of the relationship between the mind and the body, the body must be first "tamed" through self-discipline and purified by various means (see Hatha Yoga). A good level of overall health and psychological integration must be attained before the deeper aspects of yoga can be pursued. Humans have all sorts of addictions and obsessions and these preclude the attainment of tranquil abiding (meditation). Through restraint (yama) such as celibacy, abstaining from intoxicants, and careful attention to one's actions of body, speech and mind, the human being becomes fit to practice meditation. This yoke that one puts upon oneself (discipline) is another meaning of the word yoga.
Every thought, feeling, perception, or memory you may have causes a modification, or ripple, in the mind. It distorts and colors the mental mirror. If you can restrain the mind from forming into modifications, there will be no distortion, and you will experience your true Self. - Swami Satchidananda
Patanjali's Yoga Sutras begin with the statement yogas citta-v?tti-nirodha? (1.2), "Yoga limits the oscillations of the mind". They go on to detail the ways in which mind can create false ideations, and advocate meditation on real objects. This process, it is said, will lead to a spontaneous state of quiet mind, the "Nirbija" or "seedless state", in which there is no mental object of focus.
Practices that serve to maintain for the individual the ability to access this state may be considered Raja Yoga practices. Thus Raja Yoga encompasses and differentiates itself from other forms of Yoga by encouraging the mind to avoid the sort of absorption in obsessional practice (including other traditional yogic practices) that can create false mental objects.
In this sense Raja Yoga is called the "king among yogas": all yogic practices are seen as potential tools for obtaining the seedless state, itself considered to be the starting point in the quest to cleanse Karma and obtain Moksha or Nirvana. Historically, schools of yoga that label themselves "Raja" offer students a mix of yogic practices and (hopefully or ideally) this philosophical viewpoint.