Yoga Sūtra Chapter Three verse 2
tatra pratyaya-ekatānatā dhyānam ||2||
There, the continuity of psychic activity is meditation.tatra - therepratyaya - psychic activityekatānatā - continuitydhyāna - meditation
Commentaries and ReflectionsT Krishnamacharya:TKV Desikachar:
“Prāṇāyāma leads to this.
Pratyāhāra, to see without the senses distracting or pulling the mind, and
Dhāraṇā, to see without the mind losing itself, because of colouring or expectations.
Dhyānam arises out of this.”
“Perhaps the best explanation of Dhyāna is given by Patañjali in the Yoga Sūtra Chapter Three verses One and Two, where he states that one must first fix the question (Dhāraṇā) and then link to it (Dhyāna).
One who is not able to fix the question is not able to succeed in Dhyāna.”