vipaśyanā

(Sanskrit, insight; Pāli, vipassanā). One of the two main types of meditational technique taught in Buddhism, the other being śamatha, or calming meditation. The technique leads to the direct personal apprehension and verification of the truth of Buddhist teachings, such as the cognition that all formations (saṃskāra) bear the ‘three marks’ (trilakṣaṇa), namely that they are impermanent (anitya), without self-essence (anātman), and sorrowful (duḥkha). This insight leads to entry into the supermundane paths and to nirvāṇa. Vipaśyanā thus leads to an intellectual understanding of doctrine, in contrast to śamatha which leads to a transic state of rapt absorption. It is normally recommended, however, that the two techniques be developed in tandem, since insight is hard to attain if the mind is distracted.

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

vipaśyanā [vipassanā] insight. The unique feature of the Buddhist meditative development lies in its two approaches, namely, calm [śamatha] and insight [vipassanā]. These two have their own techniques and subjects. Vipaśyanā means seeing all things as they are. It denotes insight, intuitive cognition of the three characteristics of existence, namely, truth of the impermanence, suffering, and egolessness of physical and mental phenomena. It leads to direct realisation of truth.     In the Mahāyāna, vipaśyanā stands for an analytical examination of the nature of things. Such investigation leads to insight into the true nature of the world, that is, emptiness. It penetrates into the ultimate reality of all phenomena. Vipaśyanā is a name for a technique which prevents arising of passions. Insight wisdom enables the aspirant to eradicate the defilements once for all; it directly and perfectly comprehends the ultimate reality of nature and puts an end to suffering in life. The light of insight wisdom surpasses any other light. The eradication of defilements, that takes place while attaining the highest level of insight wisdom, is compared to complete destruction of the roots of a tree so that it will never grow again. The insight wisdom is an outcome of the insight meditation.     The insight meditation is to be developed on the following lines(1) Discernment of the form (rūpa),
(2) Discernment of the mind (nāma),
(3) Contemplation on both (nāmarūpa),
(4) Both viewed as conditioned (pratītyasamutpāda),
(5) Application of the three characteristics, namely, impermanency, suffering and no-soul.

     The stages of development of insight are described in terms of the nine insight-knowledges [vipassanā-ñāṇa] which form the sixth stage of purification. The insight is of both mundane and supramundane levels. The supramundane insight is of three kinds depending on its association. The three kinds of insight are the one that is associated with one of the four supramundane path, the one associated with one of the fruitions of the paths, the one regarding the extinction of consciousness [nirodha samāpatti].

Buddhānusmṛti - A Glossary of Buddhist Terms

Dīgha Nikāya. Mahāparinibbāna, Mahāsatipaṭṭhāna. Majjhima Nikāya. Ānāpānasati, Dhātuvibhaṅga, Kāyagatāsati, Mahāsaḷāyatanika, Mahāvedalla, Saḷāyatanavibhaṅga. Mahāyāna-Sūtrālaṅkāra. XI. 55; XII. 78; XIV. 90.