Not-self is the third of the Three Marks of Existence, and is also referred to as emptiness of self, or just Emptiness. It can be understood intellectually when examined through the lenses of impermanence and dependent co-arising. It can also be directly experienced when we forget ourself.

Sources: There is No Suffering by Master Sheng Yen, Dharma Drum 2001, and What makes you not a Buddhist, by Dzongsar Jamyang Khyentse, Shambhala 2008.