Kuan-yin  (Japanese, Kannon). One form of the Chinese name assigned to the Bodhisattva of compassion (karuṇā), Avalokiteśvara. This Chinese form means ‘to hear or regard the sounds’, and is a contraction of Kuan Shih Yin ‘to hear or regard the sounds of the world’, indicating the Bodhisattva's ability to hear the cries of all beings in need or trouble. In the Heart Sūtra, he is given the name Kuan Tzu Tsai. He is one of the most popular objects of devotion and reverence in east Asian Buddhism. The locus classicus for Kuan-yin's major attributes and functions is the seventh fascicle of the Lotus Sūtra, in which the Bodhisattva proclaims his all-embracing compassion and willingness to act on behalf of all suffering beings. He tells the assembly that if anyone is in any need or trouble, whether shipwreck, threat of bandits, storms, or other perils, all they need do is call upon his name single-mindedly and he will deliver them. In addition, he will grant the requests of all who pray to him. In particular, he promises to grant a child to any woman who prays to him for one, and the image of the ‘child-granting Kuan-yin’ has become especially popular. Finally, he says that he will assume any form in order to conform to the expectations and inclinations of anyone in order to teach them the Dharma and convert them: he might manifest as a Buddha, a high celestial Bodhisattva, a monk, a nun, a layman or laywoman, or even a prostitute if required. In addition to these characteristics found in the Lotus Sūtra, the Pure Land traditions in China and Japan revere Kuan-yin as one of the Three Holy Ones of Sukhāvatī. As described in the Meditation Sūtra, the Larger Sukhāvatī-vyūha Sūtra, and the Smaller Sukhāvatī-vyūha Sūtra, the Buddha Amitābha presides over this Pure Land, and is assisted by the two Bodhisattvas Avalokiteśvara and Mahāsthāmaprāpta.

Kuan-yin's ability to appear in any form needed has led to a profusion of iconographic representations. The Bodhisattva can be represented as both male and female as need and occasion demand. The greatest number of variations appear within esoteric scriptures, maṇḍalas, and images, in which he can appear in normal human shape, or with any number of heads, eyes, and arms, and also in independent guises with different names such as Chun-t'i. In one common practice, the devotee recites the Great Compassion Mantra (Chinese, Ta pei chou) while visualizing 108 separate forms of the Bodhisattva in sequence. The Bodhisattva's broad compassion (karuṇā), all-embracing vows, ability to manifest in various forms, and easy accessibility have all served to make him the most widely called-upon source of help not only in east Asian Buddhism, but in the folk beliefs of all regions as well.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)