Definition: Nembutsu is the Japanese name of the chant that is the central practice of Pure Land Buddhism. In Chinese, this chant is "Na-mu A-mi-to Fo" (Hail, Amitabha Buddha). The same chant in Japanese is "Namu Amida Butsu." Amitabha Buddha is The Buddha of Sukhavati, The Western Paradise, also called the Pure land. Pure land Buddhists chant Amitabha's name to be reborn in the Pure land, which brings one closer to Nirvana.
For the person who is seeking salvation, the Nembutsu means "Save me, Amida Buddha." It means that the seeker is opening her/himself to Amida's saving Light and Compassion. It is the seeker responding to Amida's Call and opening him/herself to Amida Buddha's Other Power.