Dhyāna. Sanskrit term meaning "meditation" (Pali form is Jhāna) and representing a notion central to the practice of Hinduism, Buddhism, and Jainism. In its penultimate stage, it leads to the final Samadhi, absorption. It is fully achieved only after passing through a series of preliminary mental states, and it is accompanied by a particular experience of the enhancement of one's inner vitality. The word itself is that from which comes the Chinese word Ch'an, the name of a Chinese mystical Buddhist school, and the Japanese word Zen.

Geddes MacGregor, Dictionary of Religion and Philosophy, New York: Paragon House, 1989

dhyāna

(Sanskrit, trance, absorption; Pāli, jhāna). A state of deep meditative absorption characterized by lucid awareness and achieved by focusing the mind on a single object (see citta-ekāgratā). A prerequisite for its attainment is the elimination of the five hindrances (nīvaraṇa). A scheme of eight stages of dhyāna was gradually evolved, with four lower assigned to the rūpa-dhātu and four higher ones assigned to the ārūpya-dhātu. In dhyāna all sense-activity is suspended, and as the meditator passes from the lower to the higher levels, mental activity becomes progressively more attenuated. Thus, in the first dhyāna, conceptualization (vitarka) and reflection (vicāra) occur, but in the second they do not. In the fifth dhyāna various supernormal powers can be attained (see ṛddhi). The names of the Ch'an and zen schools are both derived from the word dhyāna.

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

dhyāna n. meditation, thought, reflection, (esp.) profound and abstract religious meditation, (○nam āpad, ā-√sthā or ○naṃ-√gam, to indulge in religious mṭmeditation) ChUp. Mn. MBh. Kāv. &c. (with Buddhists divided into 4 stages MWB. 209 Dharmas. lxxii
• but also into 3 ib. cix)
• mental representation of the personal attributes of a deity W.
• insensibility, dulness Bhpr.
• (○ná), m. N. of a partic. personification MaitrS.
• of the 11th day of the light half in Brahma's month Pur.

Sanskrit-English Dictionary, by M. Monier William

dhyāna: Meditation, reflection. Only the first four samāpatti-s pertaining to the rūpa-dhātu can be called dhyāna, because in these four meditations, śamatha and vipaśyanā existevenly. dṛṣṭa-dharma-sukha-vihāra

Sarvastivada Abhidharma, Sanskrit-English Glossary, by Bhikkhu KL Dhammajoti