We are living in a dynamic world, where every particle is in constant flux. We cannot imagine a state where there is no action or movement. Even the apparent state of inaction reveals intense activity when analyzed scientifically. Still, there is an urge in man to be calm and silent without any activity, which induces him to go into the state of deep sleep every day. Meditation is an attempt to reach that state consciously, gradually reducing the number of thoughts and finally retaining only one thought in the mind.
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 his mind is concentrated, all his thoughts would centre on electricity alone. But in meditation there should be one subject and one thought related to that. Regarding this special kind of concentration,
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.
Source - Excerpts from article titled 'Meditation According to Hinduism' by Swami Nityasthananda published in the May 2005 issue of Prabuddha Bharata magazine.