Yogasūtra I.41 - kṣīṇa-vṛtteḥ-abhijātasya-iva maṇeḥ-grahītṛ-grahaṇa-grāhyeṣu tatstha-tad-añjanatā samāpattiḥ |

When the mind is devoid of distractions, it is possible for all mental processes to be involved in the object of enquiry. 

When distractions [of the mind] are settled, [the mind is] similar to a flawless diamond, reflecting only what is placed before it, be it the Perceiver, the process of perception or the object of perception. This is Samāpatti, the state of mental absorption.  

In the beginning, all mental activities except sleep are involved in the perception of an object.  But gradually, only those needed for correct flawless comprehension remain. 

Samāpatti is a synonym for Samādhi. This is the first level of Samādhi, and a foundation for the others that will follow.