Sri Adi Shankaracharya gives a vivid description of upasana in his commentary on the Bhagavad Gita: ‘Upasana, or meditation, means approaching an object of meditation as presented by the scriptures, making it an object of one’s own thought, and dwelling on it uninterruptedly for long by continuing the same current of thought with regard to it - like a stream of oil poured from one vessel to another.’
The analogy of the stream of oil is very appropriate. When we pour oil from one vessel to another, there will be a constant flow of oil without any sound or splash. But when we pour water in similar fashion there is so much of noise and splash all around. If the current of thought flows towards the object of meditation in an uninterrupted stream without this kind of restlessness, that state is called meditation.
This state is reached only after one has passed through two other stages of meditation. pratyahara and dharana.
Pratyahara consists in making the mind free from the clutches of the senses. The mind is always running after sense objects. When we see a particular object or hear a particular sound, the mind immediately grabs it and starts building a castle of thoughts. Same is the case when a particular thought arises in the mind. When we sit for meditation, the mind constantly goes away from the object of meditation, drawn by the objects of the senses. We withdraw the mind from these and fix it on the object of meditation. This withdrawal of the mind is called pratyahara.
But the mind refuses to remain steady and starts wandering in the world of the senses. Again and again we withdraw it from the senses, and this struggle goes on for a long time, after which the mind becomes steadier and we are able to fix it on the object of meditation. This stage is called dharana. The object of meditation can be the divine form of our chosen deity, or some sound like the pranava, or a particular centre of consciousness like the heart or the region between the two eyebrows, and so on. When the mind remains fixed on the object of meditation for a definite length of time, without being disturbed by any other thought, and the object of meditation becomes steady and vivid, then the mind is said to be in the state of meditation.