The union of the individual soul with the universal soul is said to be yoga.

Yoga is no less than the stable state of consciousness.

It is nothing but a suspension or pause in the movements of thought waves.

See what Lord Krishna says in the Bhagavad Gita.

Taking the centre body as a plumb line. He wants us to hold the right & left sides of the body evenly with a determined effort. He advises us to keep the centre of the crown of the head, the centre of the throat and the centre of the perineum, in one line and learn to adjust and bring the sides of the body parallel to each other.

As the body is held stable in this practice, fix your gaze on the tip of the nose.

Yoga is a way to maintain an even temper in ones word, work and thought.

The meaning of yoga is to restrain oneself from all bonds of pain & sorrow.

Sutra (1.1) It is a prayer. It also suggests beginning yoga with a disciplined frame of mind. Consciousness is affected more often by emotional upheaval than by intellectual deficiencies.   

Sutra (1.2) Yoga begins with the mind, as this is the part of the consciousness that comes into contact with the objects and creates the feelings. 

Yoga is a mental discipline for restraining the fluctuations of thoughts so that consciousness is kept in an unoscillated, steady, and stable state. 

Coordinating the intellect of the head with the intelligence of the heart achieves integration between the two.

I (Iyengar) believe that yoga is the union of these two branches of  intelligence.

Sutra (1.33) The spiritual heart is the seat of consciousness. As such the four chambers of the heart are physiological, psychological, mental and spiritual. The emotional divisions that correspond with these four chambers of the heart are friendliness, compassion, gladness and indifference toward pleasure&pain, virtue&vice. Through experience these have to be harmonized.

One on Patanjali’s main concerns is to bring a harmonious union between the intellect of the head and the intelligence of the heart.

This is the heart of Patanjali’s Yoga.