The term Rāja Yoga is a retronym, introduced in the 15th century Hatha Yoga Pradipika to distinguish the school based on the Yoga Sutras from the new current of Hatha Yoga. The term was later used to describe the entirely unrelated meditation practice of the Brahma Kumaris involving the focus of one's mind and surrender to a channelled entity they believe to be the Supreme Soul.
Raja Yoga is sometimes referred to as (eight-limbed) yoga because there are eight aspects to the path to which one must attend. This is not to be confused with the Ashtanga Vinyasa Yoga of K. Pattabhi Jois.
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
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 referred to as 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.
The eight limbs of Ashtanga Yoga are:
They are sometimes divided into the lower and the upper four limbs, the lower ones - from yama to pranayama - being parallel to the lower limbs of Hatha Yoga, while the upper ones - from pratyahara to samadhi - being specific for the Raja yoga. The upper three limbs practiced simultaneously constitute the Samyama.
The scientific basis of the Yama instructions may be explained in this way.
The five directives of Yama lay down behavioral norms as prerequisites for elimination of fear and angst and contribute to a tranquil mind. Nonviolence (ahimsa), truthfulness (satyavachana), non-stealing (astheya), child-like behavior (brahmacharya), and moderation in setting goals (aparigraha), prevent situations that evoke fear in human interactions and contribute to peace of mind.
Ahimsa (non-violence) helps to avoid conflicts with fellow human beings that normally occur because of competition for eating and mating. Hurting a person physically or mentally leads to fear of retaliation which disturbs peace of mind.
Satyavachan (truthfulness) helps peace of mind by eliminating fear of discovery of lies. Untruth implies maintaining two versions of an event, possibly in the same set of neurons, thus causing confusion and mental disturbance.
Astheya (non-stealing) eliminates the anxiety of discovery of theft, potential reprisal by the owner and/or punishment by state. Brahmacharya (celibacy or stage of life devoted to seeking knowledge) averts conflict associated with search for mates, eliminates fear of strife, enhances scholarship and upgrades equanimity. Aparigraha (moderation in setting goals) prevents angst of potential failure, promotes self-control and poise.
He who practises meditation without ethical perfection, without the practice of Yama-Niyama cannot obtain the fruits of meditation. Purify your mind first through the practice of Yama-Niyama. Then practice regular meditation. Then you will attain illumination.
Science underslying Niyama can be presented as follows.
Niyama unlike Yama prescribes mental exercises to train the mind to control emotions: (Saucha), (Santosha), (Tapaha), (Svadhyaya) and (Ishwara pranidhanani).
Saucha (Purity of thought) requires active monitoring of the mind from being obsessed with material or corporeal desires.
Santosha (Contentment) prevents desire to obtain, experience and /or accumulate objects of pleasure.
Tapaha (Austere or ascetic life style), by practicing self-denying and austere life style controls fondness for desire-generated emotions.
Svadhyaya (Study about self) enlightens the true nature of human nature and facilitates healthy thoughts.
Ishwara pranidhanani (Surrender of ego to God) makes it easier to be not self-centered by aiming at higher goals.
Thus Niyama instructions prepare the mind to control animal (reptilian brain- generated) and human (cerebral) emotions.
Asana and Pranayama recognize the intimate connection between body and mind. Asana are a series of gentle physical exercises designed to keep muscles in tone, joints supple and nerves in tune.
Pranayama is the prelude to concentration of mind. Pranayama brings the involuntary breathing cycle under voluntary control by regulating inhalation, exhalation and holding breath static either in the lungs or out of them for periods which could be long or short. Active regulation of breathing naturally requires concentrating on the process, a prelude to control of thinking process the final goal of Raja yoga.
The objective of Prathyahara is to disrupt the communication from the sense organ to the brain. Light, sound, smell etc. stimuli received by the sense organs are blocked from the brain centers and thus could not distract from concentration of the mind.
The mind passes into many conditions or states as it is made up of three qualities-Sattva, Rajas and Tamas. Kshipta (wandering), Vikshipta (gathering), Mudha (ignorant), Ekagra (one-pointed), and Nirodha (contrary) are the five states of the mind.
By controlling the thoughts the Sadhaka attains great Siddhis. He becomes an adept. He attains Asamprajnata Samadhi or Kaivalya. Do not run after Siddhis. Siddhis are great temptations. They will bring about your downfall. A Raja Yogi practices Samyama or the combined practice of Dharana, Dhyana and Samadhi at one and the same time and gets detailed knowledge of an object.
Control the mind by Abhyasa (practice) and Vairagya (dispassion). Any practice which steadies the mind and makes it one-pointed is Abhyasa. Dull Vairagya will not help you in attaining perfection in Yoga. You must have Para Vairagya or Theevra Vairagya, intense dispassion.
Samadhi is of two kinds:
In Savikalpa or Sabija, there is Triputi or the triad (knower, known and knowledge). The samskaras are not burnt or freed. Savitarka, Nirvitarka, Savichara, Nirvichara, Sasmita and Saananda are the different forms of Savikalpa Samadhi. In Nirvikalpa Samadhi, Nirbija Samadhi or Asamprajnata Samadhi there is no triad.
A Bhakta gets Bhava-Samadhi, a Jnani gets Badha-Samadhi, a Raja Yogi gets Nirodha Samadhi.
In a lay-persons terms, Samadhi would be described as enlightenment.