From Zenwiki

means "separation" as in a-sun-der, sun-dry. It starts from karma, action or function, of sense organs (eyes, ears, nose, tongue, body, and mind) and motor organs (hands, feet, body, bowels, etc.), separating "self" from "other," the fundamental, cardinal, delusion of the independent, eternal ego (I, Ich, Je, Aham, Wo, Ware, etc.). It develops into liking (desire, craving, etc.) and disliking (hatred, anger, etc.), together called the Triple Poisons of delusion, desire, divisiveness. As these are developed by karmas, they are countered by sitting still (Zazen) and stilling karma (nirvana), leading to holiness (wholly wholesomeness), where one can see no self "substance" or "entity" from no self-sameness (impermanence) and self-sovereignty (suffering) from the Dharma (Law/truth)of Dependent Co-origination.