Trataka (in Sanskrit, n., त्राटक, trāṭaka, to look, or to gaze) is the practice of staring at some external object. It is used in yoga as a way of developing concentration, strengthening the eyes, and stimulating the Ajna chakra.
In its first stage, the practitioner fixes his attention on a symbol or yantra, such as the Om symbol, a black dot, or the image of some deity, and stares at it, paying attention to each thought and feeling as it arises, and letting them go, so that the mind is completely absorbed in the symbol. The practice continues until the eyes begin to water, at which point they are closed, and relaxed.
The second stage is staring at a candle flame. The practice is the same up until the eyes begin to water, after which the eyes are closed, and the yogi tries to concentrate on the after image, and hold it for as long as possible. At first, it will be a real after-image, but later, it will exist only in the mind’s eye, and the exercise in concentration comes from trying to maintain it there for a long period of time.
Trataka is supposedly the technique which sadhakas use to develop psychic powers. Trataka on the Sun or one’s own mirror image is considered to be extremely powerful, but without a guru‘s assistance it can be dangerous to try these.
Definition courtesy Wikipedia.