Union of subject and object becomes samadhi.

Verse 3.3

तदेवार्थमात्रनिर्भासं स्वरूपशून्यमिवसमाधिः ॥३॥

tadeva-artha-mātra-nirbhāsaṁ svarūpa-śūnyam-iva-samādhiḥ ॥3॥

When the object of meditation engulfs the meditator, appearing as the subject, self-awareness is lost. This is samadhi.

Verse 3.4

त्रयमेकत्र संयमः ॥४॥

trayam-ekatra saṁyamaḥ ॥4॥

These three together - dharana, dhyana and samadhi - constitute integration

or samyama.

When the attentive flow of consciousness merges with the object of meditation, the consciousness of the meditator, the subject, appears to be dissolved in the object. 

This union of subject and object becomes samadhi.

When the object of contemplation shines forth without the intervention of one's own consciousness, dhyana flows into Samadhi.

Uninterrupted flow of attention dissolves the split between the object seen and the seer who sees it. Consciousness appears to have ceased, and to have reached a state of silence. It is devoid of 'I', and merges into the core of the being in a profound state of serenity.

 In samadhi, awareness of place vanishes and one ceases to experience space and time.

Samyama is a technical word defining the integration of concentration (dharana) , meditation (Dhyana) and absorption (samadhi).

 In samyama the three are a single thread, evolving from uninterrupted attention to samadhi.

Dharana is single-pointed attention. It modifies into dhyana by being sustained in time whilst dissolving its one-pointed character implicit in the word 'concentration'. When it becomes all-pointed, it leads to total absorption (samadhi) . 

Continuous prolongation of these three subtle aspects of yoga thus forms a single unit, called samyama. Samyama, is a state of immobility, and a samyami is one who subdues his passions and remains motionless.

The following analogy shows the organic relationship between dharana, dhyana and samadhi. When one contemplates a diamond, one at first sees with great clarity the gem itself. Gradually one becomes aware of the light glowing from its centre. As awareness of the light grows, awareness of the stone as an object diminishes. 

Then there is only brightness, no source, no object. When the light is everywhere, that is samadhi.

As dharana is external to dhyana, dhyana to samadhi, samadhi to samyama and samyama to nirbija samadhi, so the mind is external to intelligence, intelligence to consciousness and consciousness to the seer.

Dharana ,dhyana and samadhi intermingle to become samyama, or integration.

The intermingling of mind, intelligence and consciousness is samyama of the three. The vision of the seer is equivalent to nirbija samadhi.

PATANJALI YOGA SUTRAS