cakkh' āyatana: 'the base "visual organ" ' (s. āyatana).
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-ppatigha).... (Vibh. II). Similar is the explanation of the four remaining physical sense-organs.
cakkhāyatana, cakkhvāyatana(cakkhu 眼) + (āyatana 處): n. 眼的處