It refers to wisdom that has been discovered by a seeker through self effort. It teaches us to seek wisdom in the depths of our own soul.

Scriptural definitions:

Yogah Chitta Vritti Nirodha (Patanjali Yoga Sutra, Chapter 1, Sutra 2)

Yoga is a slow, gradual process of bringing the mind and all its activities to a state of stillness. When the mind becomes still, pure consciousness and pure energy unite and become one leading to knowledge of the Self.

Samatvam Yoga Uchyate (Bhagavad Gita, Chapter 2, Shloka 1 - 48)

Yoga is the science of training the mind to remain even in pleasure or pain, gain or loss, defeat or victory, success or failure, and be able to rise above them.

Yogah Karmasu Kaushalam (Bhagavad Gita, Chapter 2, Shloka 1 - 50)

Yoga is the science of performing right actions in accordance with the universal Law of Karma. It is the science of selfless activities without the notion of being a doer and without seeking the fruits of the activities, thus freeing ourselves from the bondage of the law and the consequent cycles of births and deaths.

Dhukha Samyoga Viyogam Yoga Samjnitam (Bhagavad Gita, Chapter 6, Shloka 1 - 23)

The pain free state is yoga. When the mind is free of all pain producing factors, that pain free state of the mind is called yoga.
 

Much of the knowledge on this website has to be attributed to Swamiji Dharmananda, with whom I have studied at Ved Niketan Ashram Rishikesh (India). In fact, some of the texts have been quoted from a the book titled "Advanced Yoga Study" authored by Swamiji. You can find this comprehensive book on yoga philosophy on his website here http://www.swamidharmananda.com/. There are also some videos from his lectures available on youtube if you do a quick search. To him, I am eternally grateful for having helped me start this journey.

If you are interested in studying yoga in an ashram, here is a short list of ashrams in Rishikesh which offer full yoga programmes.

There are plenty of teachers in Rishikesh offering yoga retreats, teacher training courses and daily walk-in classes. Here are some yoga retreat centers you might like to check out if you prefer to study yoga outside an ashram environment. This is just a list, not a recommendation, so do check it out personally before committing to one. Otherwise a google search will bring up many centers as well.