Patanjali Yoga Sutra — ततः पुनः शान्तोदितौ तुल्यप्रत्ययौ चित्तस्येकाग्रता परिणामः॥3.12॥

tataḥ punaḥ śāntoditau tulyapratyayau cittasyaikāgratā pariṇāmaḥ॥

Hoisting of completely identical thought waves in very quick rhythmic succession is ekāgratā pariṇāmaḥ.

The condition is similar to shooting of only one still object, with a movie camera, several hundred times and projecting the outcome on a screen. Though the film-roll runs, we view only one object on screen uninterruptedly and we never feel the movement of discrete images in film. Similarly, if we rise completely identical thought waves in very quick rhythmic succession, as they become indistinguishable, identical impressions are created and sādhaka feels that he is focusing on only one thought. This propensity of mind is named as ekāgratā pariṇāmaḥ.

Gītā says:

सुखमात्यन्तिकं यत्तद् बुद्धिग्राह्यमतीन्द्रियम्।

वेत्ति यत्र न चैवायं स्थितश्चलति तत्त्वतः॥गीता ६.२१॥

Sukhamātyantikaṃ yattadbuddhigrāhyamatīndriyam|

vetti yatra na caivāyaṃ sthitaścalati tattvataḥ॥Gītā 6.21॥

In such a state, Yogi experiences unending bliss which is transcendental and beyond understandable through the senses and never moves from this eternal reality.