Atha Samadhi Padah
The Yoga Sutra is the most important text on Yoga. It is divided into four chapters: Samadhi Pada (chapter on samadhi), Sadhana Pada (chapter on practice), Vibhuti Pada (chapter on powers) and Kaivalya Pada (chapter on liberation).
The first chapter, the Samadhi Pada, is the chapter about samadhi. In verse 3 and 4, Patanjali defines (two types of) samadhi as a steady state of mind, one in which the mind is either totally concentrated on one object, or one in which the mind's activities totally cease, in which the mind is totally empty of content. When this happens, consciousness, or the Self, the Witness or Seer is established in its essence, while at other times it appears as though our essential nature is merged or fused with the content of the mind.
In other words, in "normal" experience, consciousness is merged with the mind and its contents/activities, while in samadhi, consciousness is isolated, disunited, experienced separately from the mind, its contents and activities. Although yoga is often defined as "union", the yoga of Patanjali is however, defined as dis-connection or dis-union of consciousness from mind, while "normal" experience is explained as "union" or merging of Self with experience.
The Samadhi Padha is written with instructions or indications for the advanced, mature or "ripe" student - with a few simple ideas, Patanjali indicates the immediate means to realizing the state of yoga. By contrast, the second chapter, the Sadhana Pada is written for the student with a distracted mind - here he explains practices, including the eight limbs of Ashtanga Yoga as a means to overcoming the obstacles to meditation and yoga.