Samadhi (Sanskrit: समाधि) literally means “putting together, joining, combining with, union, harmonious whole, trance”.
Samadhi is oneness with the subject of meditation. There is no distinction, during the eighth limb of yoga, between the actor of meditation, the act of meditation and the subject of meditation. Samadhi is that spiritual state when one’s mind is so absorbed in whatever it is contemplating on, that the mind loses the sense of its own identity. The thinker, the thought process and the thought fuse with the subject of thought. There is only oneness, samadhi. ~ Wikipedia
The first seven limbs of the eight limbs of ashtanga are preparations for Samadhi.