All kinds of interesting definitions of the word Yoga exist. To Patanjali its chiita vritti nirodh, to Krishna its karmasu kaushalam and of course Ramanuja would make it Bhakti :)
Lets see what the Champion Shankara says in his classic aparoksha anubhuti on the eight limbs of yoga :)
Its interesting to see how samadhi becomes Knowledge !
102-103. The steps, in order, are described as follows: the control of the senses, the control of the mind, renunciation, silence, space, time, posture, the restraining root (Mulabandha), the equipoise of the body, the firmness of vision, the control of the vital forces, the withdrawal of the mind, concentration, self-contemplation and complete absorption.
104. The restraint of all the senses by means of such knowledge as “All this is Brahman” is rightly called Yama, which should be practiced again and again.
105. The continuous flow of only one kind of thought to the exclusion of all other thoughts, is called Niyama, which is verily the supreme bliss and is regularly practiced by the wise.
106. The abandonment of the illusory universe by realizing it as the all-conscious Atman is the real renunciation honored by the great, since it is of the nature of immediate liberation.
107. The wise should always be one with that silence wherefrom words together with the mind turn back without reaching it, but which is attainable by the Yogins.
108-109. Who can describe That (i.e., Brahman) whence words turn away ? (So silence is inevitable while describing Brahman). Or if the phenomenal world were to be described, even that is beyond words. This, to give an alternate definition, may also be termed silence known among the sages as congenital. The observance of silence by restraining speech, on the other hand, is ordained by the teachers of Brahman for the ignorant.
110. That solitude is known as space, wherein the universe does not exist in the beginning, end or middle, but whereby it is pervaded at all times.
111. The non-dual (Brahman) that is bliss indivisible is denoted by the word ‘time’, since it brings into existence, in the twinkling of an eye all beings from Brahman downwards.
112. One should known that as real posture in which the meditation on Brahman flows spontaneously and unceasingly, and not any other that destroys one’s happiness.
113. That which is well known as the origin of all beings and the support of the whole universe, which is immutable and in which the enlightened are completely merged … that alone is known as Siddhasana (eternal Brahman).
114. That (Brahman) which is the root of all existence and on which the restraint of the mind is based is called the restraining root (Mulabandha) which should always be adopted since it is fit for Raja-yogins.
115. Absorption in the uniform Brahman should be known as the equipoise of the limbs (Dehasamya). Otherwise mere straightening of the body like that of a dried-up tree is no equipoise.
116. Converting the ordinary vision into one of knowledge one should view the world as Brahman itself. That is the noblest vision, and not that which is directed to the tip of the nose.
117. Or, one should direct one’s vision to That alone where all distinction of the seer, sight, and the seen ceases and not to the tip of the nose.
118. The restraint of all modifications of the mind by regarding all mental states like the Chitta as Brahman alone, is called Pranayama.
119-120. The negation of the phenomenal world is known as Rechaka (breathing out), the thought, “I am verily Brahman”, is called Puraka (breathing in), and the steadiness of that thought thereafter is called Kumbhaka (restraining the breath). This is the real course of Pranayama for the enlightened, whereas the ignorant only torture the nose.
121. The absorption of the mind in the Supreme Consciousness by realizing Atman in all objects is known as Pratyahara (withdrawal of the mind) which should be practiced by the seekers after liberation.
122. The steadiness of the mind through realization of Brahman wherever the mind goes, is known as the supreme Dharana (concentration).
123. Remaining independent of everything as a result of the unassailable thought, “I am verily Brahman”, is well known by the word Dhyana (meditation), and is productive of supreme bliss.
124. The complete forgetfulness of all thought by first making it changeless and then identifying it with Brahman is called Samadhi known also as knowledge.