Here is a look at the difference between concentration and meditation based on teachings in Hindu religion.
In both, yoga and the Upanishads, meditation is considered an important means of attaining spiritual realization. Meditation is a special kind of concentration. In ordinary concentration, the mind is focused on one particular subject and there can be many divergent thoughts related to that particular subject. Here the subject is one, but thoughts are many and dissimilar. For example, if one is reading a book on electricity and if one’s mind is concentrated, all the thoughts would centre on electricity alone. There would be different thoughts, but all related to electricity only. But in meditation there should be one subject and one thought related to that.
Regarding this special kind of concentration (meditation), Swami Yatiswarananda says:
It is important to know the difference between ordinary concentration and meditation. By the word ‘meditation’ we mean dhyana or contemplation. It is not just ordinary concentration. It is a special type of concentration. In the first place, meditation is a fully conscious process, an exercise of the will. Secondly, meditation means concentration on a spiritual idea which presupposes that the aspirant is capable of rising above worldly ideas. And finally, meditation is done usually at a particular centre of consciousness. It is clear that true meditation is a fairly advanced state, attained after long practice. It is the result of long years of discipline.
If one is meditating on a particular divine form of Sri Rama at a particular centre of consciousness, say the heart, then there would be a continuous flow of the same thought representing the divine form of Sri Rama, to the exclusion of all other thoughts, even the thoughts related to Sri Rama’s qualities or his life. This continuous flow of one same thought is called meditation.
According to Patanjali, ‘An unbroken flow of thoughts of that (object of meditation) is called dhyana’. This is similar to the upasana spoken of in Vedanta. Acharya Adi Shankara gives a vivid description of upasana in his commentary on the 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.
Source – article titled ‘Yoga and Upanishads’ by Swami Nityasthananda published in Prabuddha Bharata January 2019 issue page 54-55