The primary meaning of the word satsanga is the company of the holy, meaning, association with and becoming one with the supreme Self, which is of the essential nature of Truth. That means — to remain steady in mental modification merged in the form of Brahman (brahmakara-vritti). All ignorance and worldly bondage become annihilated if the mind remains even for a moment in that mental modification.
The secondary meaning of the word satsanga is the company of holy people, sadhus, who remain mediating on the supreme Self whose essential nature is Truth. To study the books written by the great souls who are the knower of Truth, and to read their instructions illuminated by their experiences are also termed sat-sanga. That is because it is through their instructions that union between the aspirant and the supreme Self, whose essential nature is Truth, can occur.
Source – Notes on Yoga-Vasishtha-Sara by Swami Dhireshananda - Swamiji is monk in the Ramakrishna Order