Yoga Sutras of Patanjali translated by Yogi Kalinath - Chapter 3, Verse 7
Pada 3, Sutra 7
Trayam antar-angam poorvebhyah
Compared to the previous limbs, these three are internal.
According to Rishi Patanjali, Yoga is a step-by-step process composed of eight limbs. The first five limbs: Yama, Niyama, Asana, Pranayama and Pratyahara are the external interface with the Yogi. Respectively they purify our action organs (Yama), senses (Niyama), physical temple (Asana), energy (Pranayama) and mind (Pratyaahaara—by reversing its focus from the external to the internal).
On the other hand, Samyama or constraint comprising of Dharaaranaa (concentration), Dhyaana (Meditation) and Samaadhi (Cognitive Absorption) are the internal interface with the Yogi.
Concentration sharpens the mind making it laser-like and very efficient; Meditation purifies consciousness because in the true state of Meditation the ego, the I-maker disappears; and in Cognitive Absorption the mind dissolves to reveal Atman as Sat-Cit-Anandam or Being-Consciousness-Bliss.
Yoga is a process of outer to inner dissolution of ego identification.