kasiṇa: (perhaps related to Sanskrit krtsna, 'all, complete, whole'), is the name for a purely external device to produce and develop concentration of mind and attain the 4 absorptions (jhāna q.v.). It consists in concentrating one's full and undivided attention on one visible object as preparatory image (parikamma-nimitta), e.g. a colored spot or disc, or a piece of earth, or a pond at some distance, etc., until at last one perceives, even with the eyes closed, a mental reflex, the acquired image (uggaha-nimitta). Now, while continuing to direct one's attention to this image, there may arise the spotless and immovable counter-image (paṭibhāga-nimitta), and together with it the neighbourhood-concentration (upacāra-samādhi) will have been reached. While still persevering in the concentration on the object, one finally will reach a state of mind where all sense-activity is suspended, where there is no more seeing and hearing, no more perception of bodily impression and feeling, i.e. the state of the 1st mental absorption (jhāna, q.v.).

The 10 kasiṇas mentioned in the Suttas are: earth-kasiṇa, water, fire, wind, blue, yellow, red, white, space, and consciousness. "There are 10 kasiṇa-spheres: someone sees the earth kasiṇa, above, below, on all sides, undivided, unbounded .... someone see the water-kasiṇa, above, below, etc." (M. 77; D. 33) Cf. abhibhāyatan, bhāvanā; further s. Fund. IV.

For space and consciousness-kasiṇa we find in Vis.M. V the names limited space-kasiṇa (paricchinnākāsa-kasiṇa; . . . s. App. ) and light-kasiṇa (āloka-kasiṇa).

 For full description see Vis.M. IV-V; also Aṭṭhasālinī Tr. I, 248.

Buddhist Dictionary, Manual of Buddhist Terms and Doctrines, by NYANATILOKA MAHATHERA

kṛtsna [kasiṇa] a kind of device of meditation. Kasiṇa is an external device chosen for concentration. It is an aid to concentration culminating into absorptions (dhyāna). Kasiṇa is conducive to the mystic meditation. Generally, ten kasiṇa-s are enumerated. They are the earth kasiṇa, the water kasiṇa, the fire kasiṇa, the wind kasiṇa, the blue kasiṇa, the yellow kasiṇa, the red kasiṇa, the white kasiṇa, the space kasiṇa, and the consciousness. Sometimes only eight kasiṇa-s are mentioned omitting the last two.

     Each kasiṇa is fivefold -- above, below, horizontal, non-dual, and beyond measure. When attention is focused on one particular kasiṇa object, by gazing at it constantly, a mental image is formed from the concentration on the visible image, and from the mental image is formed the counter image leading to the upacāra samādhi. This concentration ultimately culminates into the first mental absorption.

Buddhānusmṛti - A Glossary of Buddhist Terms

Aṭṭhasālinī. I. 274, 298, 303. Dīgha Nikāya. Saṅgīti. Visuddhimagga. III. 89; IV. 120-121; V. 138-144.