Advaita Vedanta is the classic thought process of nondual philosophy.
In simpler language, the core teaching is 'The Absolute (Brahman) is the only reality, the world (jagat) is an appearance (of that nondual Absolute) and the sentient being (jiva) is none other than the nondual Absolute'.
Adi Shankaracharya was the proponent of this Philosophy.
There are two steps to Advaita—first, 'neti, neti' which means 'not this, not this' leading to the realization of our spiritual nature.
Secondly, we need to see that this spiritual natural state of ours is nondual. Clarity about both steps is necessary to realize nonduality (Advaita).
Disclaimer: No intent to infringe any copyrights on publicly available resources.