Kasina: (possible relation to Sanskrit krtsna) "all, complete, whole"; a purely external meditation device. It serves to produce and develop concentration of mind for the attainment of the four absorptions (jhānas).
WARNING: Success will depend on proper virtue and motivation. It is essential to follow the Five Precepts and preferable to temporarily undertake the Eight Precepts. While temporary ordination is not for everyone, with the proper intention it can serve as a catalyst and provide the most conducive circumstances for quick success. Wholesome motivation -- establishing oneself in samadhi in order to succeed in insight -- is the only way to remain safe: Beings come to ruin by the pursuit of selfish ends. Concentration is not the goal. Concentration (along with bliss, serenity, happiness, and equanimity) is the means of empowering the mind for the quest. The goal is enlightenment, helping others, and final liberation from all suffering. Buddhism, after all, is the "marriage" of universal compassion and transcendent wisdom.
The meditation procedure consists of concentrating one's full and undivided attention on a single visible object for an extended period. This object is called the preparatory-image or "sign" (parikamma-nimitta). For example, it may be a colored disc or spot (large for the inattentive, smaller for the meticulously attentive), or a piece of reddish earth, or a pond or body of water at some distance, or a candle flame, and so on.
One adverts attention (the opposite of averting) bringing the mind to the object and sustaining attention on it. One does so until at last one perceives, even with the eyes closed, a mental reflex image. This sign is called the acquired-image (uggaha-nimitta).
Now, while continuing to direct one's attention to this external image, there may arise the spotless and immovable counterpart-image (paṭibhāga-nimitta). With it comes neighborhood-concentration (upacāra-samādhi), which has been reached by cleansing the mind of impurities by collecting it on the object and concentrating it like a laser.
While still persevering in the concentration on the object, one will finally reach a state of mind where all distracting sense-activity (external sights, sounds, and so on) is suspended; there is no more seeing and hearing, no more perception of bodily impression and sensation. This fully lucid and conscious state, with the mind "absorbed" in the chosen object of meditation, is the first jhāna.
It's "fault" is that it is not far from distracted states. It is easy to fall out of and needs a great deal of practice so that the mind comes to prefer it (and the profound bliss, joy, or rapture associated with it) over its ordinary distracted state of attending to many objects or jumping around with a short attention span, or simply jumping around with a "monkey mind" beset by the obstructing Five Hindrances.
- The ten kasiṇas mentioned in the sutras are: earth-kasiṇa, water, fire, wind, blue, yellow, red, white, space, and consciousness. "There are 10 kasiṇa-spheres: One sees the earth kasiṇa above, below, on all sides, undivided, and unbounded.... One sees the water-kasiṇa above, below, and so on" (Middle Length Discourses 77; Lengthy Discourses 33). See abhibhāyatana (the eight Stages of Mastery or powers obtained by means of the kasina exercises) and bhāvanā ("meditation").
For space and consciousness-kasiṇas, The Path of Purification (Vis.M. V) refers to limited space-kasiṇa (paricchinn-ākāsa-kasiṇa, a commentarial term for akasa-kasina) and light-kasiṇa (āloka-kasiṇa). For full description see Vis.M. IV-V and also Aṭṭhasālinī Tr. I, 248 (the commentary to the Dhammasangani).