There are very many ways to define the meaning of “Yoga”.
Patañjali begins the Yoga Sutras with the “classical” definition of Yoga.
What is Yoga?
Yoga Sutras 01:02
“yogaḥ citta-vṛtti-nirodhaḥ”
“Yoga is the settling of the mind into silence”
Alistair Shearer
“Complete mastery over the roaming tendencies of the mind is yoga”
Pandit Rajmani Tigunait
“The restraint of the modifications of the mind-stuff is yoga”
Swami Satchitananda
“Yoga is the containment of movement in the psyche”
Paul Harvey
So we can see that Yoga is a mental process and a mental state.
It is a process whereby the mind progressively comes to a clear/calm/contained/tamed/mastered state.
And it is that state of clear/calm/contained/tamed/mastered mind.
Yoga is both journey and destination.
… and then …
Yoga Sutras: 01:03
“tadā draṣṭuḥ svarūpe-avasthānam”
“When the mind is settled, we are established in our essential nature, which is unbounded consciousness.”
Alistair Shearer
“Then the Seer [Self] becomes established in its essential nature.”
Pandit Rajmani Tigunait
“Then the Seer [Self] abides in his own nature.”
Swami Satchitananda
“Then the Seer’s own character remains.”
Paul Harvey
We reflect our True Self, our True Nature, our Deepest Reality, and It is fully and completely present and established in our human life, shining without block or hindrance.
One of the names Patañjali gives this Truth is the “Seer.”
Note that there is no mention of the physical postures, or any other specific techniques.
The equating of the word “yoga” with bending and stretching is a very modern phenomenon.
Patañjali goes on to offer a very broad spectrum of possible processes, principles and practices that can constitute the journey, and to define the destination and its results.
If we are not working to quieten/calm/master the mind in order to progress spiritually we are not, in the classical sense of the word, practising yoga.