SAMĀDHI Skt., lit. “establish, make firm.”
[Hinduism] A state of consciousness that lies beyond waking, dreaming, and deep sleep, and in which mental activity ceases. It is a total absorption in the object of meditation. If that object is God or the Absolute, the result is union. There are various stages of samādhi, of which the highest is nirvikalpa-samādhi.
[Buddhism] [Zen] (Jap., sanmai or zammai); collectedness of the mind on a single object through (gradual) calming of mental activity. Samādhi is a non-dualistic state of consciousness in which the consciousness of the experiencing “subject” becomes one with the experienced “object”—thus is only experiential content. This state of consciousness is often referred to as “one-pointedness of mind”; this expression, however, is misleading because it calls up the image of “concentration” on one point on which the mind is “directed.” However, samādhi is neither a straining concentration on one point, nor is the mind directed from here (subject) to there (object), which would be a dualistic mode of experience.
The ability to attain the state of samādhi is a precondition for absorption (dhyāna).
Three supramundane (lokottara) types of samādhi are distinguished that have as their goal Emptiness (Shūnyatā), the state of no-characteristics (animitta) and freedom from attachment to the object, and the attainment of Nirvāna. Any other form of samādhi, even-in the highest stages of absorption, is considered worldly.
Source: The Encyclopedia of Eastern Philosophy and Religion: Buddhism, Hinduism, Taoism, Zen. Shambhala Publications, Inc.
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