Ayatana

Āyatana:
1. Spheres, is a name for the four formless absorptions; see: jhāna 5-8.
2. The 12 sources or bases on which depend the mental processes, consist of five physical sense-organs and consciousness, being the six internal ajjhattika sources; and the six objects, the so-called external bāhira sources - namely:
#eye, or visual organ and visible object
#ear, or auditory organ and sound, or audible object
#nose, or olfactory organ and odour, or olfactive object
#tongue, or gustatory organ and taste, or gustative object
#body, or tactile organ and body-contact, or tactile object
#mind-base, or consciousness and idea or mental-object manāyatana dhammāyatana 
By the visual organ cakkhāyatana is meant the sensitive part of the eye cakkhu-pasāda (retina) built up of the four elements... responding to sense-stimuli sa-ppatigha. Vibh. II. Similar is the explanation of the four remaining physical sense-organs.
The source-of-mind manāyatana is a collective term for all consciousness whatsoever, and should therefore not be confounded with the mental-element mano-dhātu see: dhātu II, 16, which latter performs only the functions of directing āvajjana attention to the sense-object, and of receiving sampaticchana the data of the sense-object. On the functions of the mind, see: viññāna-kicca
The visible object rūpāyatana is described in Vibh. II as that phenomenon which is built up of the four primary elements and appears as color and form. What is seen by-visual perception, i.e. by visual-consciousness cakkhu-viññāna are only colors and different intensities of light, but not three dimensional bodily things, which are interpretations.
The thinkable mental-object-source dhammāyatana is identical with 'mental-object-element' dhamma-dhātu, dhātu II-17 and dhammārammana see: ārammana. It may be physical or mental, past, present or future, real or imaginary.
The 5 physical sense-organs are also called abilities indriya, and of these abilities it is said in M. 43: Each of the five abilities owns a different domain, and none of them partakes of the domain of another one;... they have mind as their support... are conditioned by mental vitality (neural metabolic activity).
The 12 sense-source are fully discussed in Vis.M XV. In Yam III see: Guide, p 98f. are
the 12 terms are subjected to a logical investigation The six internal bases form the 5th link of dependent origination paticca-samuppāda 5.
==References==
Maha Thera Nyanatiloka. Manual of Buddhist Terms and Doctrines, Buddhist Publication Society, first edition 1952. 
Category:Pali terms