Source: Buddhist Dictionary, Manual of Buddhist Terms and Doctrines, by NYANATILOKA MAHATHERA
āyatana: 1. 'spheres', is a name for the four immaterial absorptions; s. jhāna (5-8).
2. The 12 'bases' or 'sources' on which depend the mental processes, consist of five physical sense-organs and consciousness, being the six personal (ajjhattika) bases; and the six objects, the so-called external (bāhira) bases - namely:1. eye, or visual organ visible object2. ear, or auditory organ sound, or audible object3. nose, or olfactory organ odour, or olfactive object4. tongue, or gustatory organ taste, or gustative object5. body, or tactile organ body-impression, or tactile object
6. mind-base, or consciousness mind-object (manāyatana) (dhammāyatana)
"By the visual organ (cakkhāyatana) is meant the sensitive part of the eye (cakkhu-pasāda) built up of the four elements ... responding to sense-stimuli" (sa-ppaṭigha).... (Vibh. II). Similar is the explanation of the four remaining physical sense-organs.
Mind-base (manāyatana) is a collective term for all consciousness whatever, and should therefore not be confounded with the mind-element (mano-dhātu; s. dhātu II, 16), which latter performs only the functions of adverting (āvajjana) to the sense-object, and of receiving (sampaṭicchana) the sense-object. On the functions of the mind, s. viññāṇa-kicca.
The visible object (rūpāyatana) is described in Vibh. II as "that phenomenon which is built up of the four physical elements and appears as color, etc." What is' seen by-visual perception, i.e. by eye-consciousness (cakkhu-viññāṇa) are colors and differences of light, but not three dimensional bodily things.
'Mind-object-base' (dhammāyatana) is identical with 'mind-object-element' (dhamma-dhātu; s. dhātu II) and dhammārammaṇa (s. ārammaṇa). It may be physical or mental, past, present or future, real or imaginary.
The 5 physical sense-organs are also called faculties (indriya, q.v.), and of these faculties it is said in M. 43: "Each of the five faculties owns a different sphere, and none of them partakes of the sphere of another one; ... they have mind as their support... are conditioned by vitality, ... but vitality again is conditioned by heat, heat again by vitality, just as the light and flame of a burning lamp are mutually conditioned."
The 12 bases are fully discussed in Vis.M. XV. In Yam III (s Guide, p 98f) the 12 terms are subjected to a logical investigation The six personal bases form the 5th link of dependent origination (paṭiccasamuppāda 5, q.v.).
Source: A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)
āyatana (Sanskrit). In Buddhist psychology, the twelve āyatanas are the six senses or modes of perception and the six kinds of object they correspond to, namely: (1) sight and colour/form (rūpa-āyatana); (2) hearing and sound (śabda-āyatana); (3) smell and scent (gandha-āyatana); (4) taste and flavours (rasa-āyatana); (5) touch and tangible objects (sparśa-āyatana); and (6) the mind and ideas (mano-āyatana). Each āyatana is thus the sphere or domain of a particular sense, and encompasses everything that can be experienced through that particular ‘sense-door’. See also ṣad-āyatana.
Source: A.P. Buddhadatta Mahathera, Concise Pali-English and English-Pali Dictionary [available as digital version from Metta Net, Sri Lanka]
āyatana : [nt.] sphere; region; sense-organ; position.
Source: Pali-English Dictionary, TW Rhys Davids, William Stede,
Āyatana (nt.) [Sk. āyatana, not found in the Vedas; but freq. in BSk. From ā + yam, cp. āyata. The pl. is āyatanā at S
-- uppāda birth of the āyatanas (see above 3) Vin
Source: 巴漢辭典 編者：(斗六) 廖文燦
āyatana(<[ā]+[yam], cp. [āyata伸展(pp.)]): n. 1.處 2.伸展
Source: Sarvastivada Abhidharma, Sanskrit-English Glossary, by Bhikkhu KL Dhammajoti
āyatana: Entrance, abode: a unique cognitive species. There are 12; the 6 internal faculties (from cakṣus to manas) and the 6 corresponding object-domains (from rūpato dharma).