Meditation is not concentration. Concentration is holding the mind to something within or outside the body; while, meditation is an unbroken flow of thoughts towards the object of concentration. It can be called prolonged concentration. Meditation is like pouring of oil from one vessel to another in a steady unbroken stream.

Samadhi or absorption is when the object of concentration and the mind of the perceiver becomes one. When Concentration, Meditation and Samadhi are brought to bear upon one subject it is called Samyam.

According to yoga sutras of Patanjali, (3.1–3.6), meditation needs to be learnt and applied step by step. The practice starts by sitting straight with erect spine, preferably in Padmasana (one can also sit on the chair) and concentrate on the breathing or on a primordial sound given by the teacher.

When the mind can be made to flow uninterruptedly towards the same object for 12 seconds, one is said to have learnt the process of concentration.

When the mind can continue in that concentration for 12 times (12 seconds × 12 i.e. 2 minutes 24 seconds), one is said to be practicing meditation.

When the mind can continue in that concentration for 12 times (12 minutes 24 seconds × 12 i.e. 28 minutes 48 seconds), one is said to be in Samadhi.

And if this lower Samadhi can be maintained for 12 times, i.e., for 5 hours 45 minutes and 36 seconds, one is said to be in Nirvikalpa Samadhi.

The mind becomes one-pointed when similar thought waves arise in succession without any gap between them.

One should remember that during meditation, the object of concentration may change in form, time and rhythm.

The whole process of meditation, therefore, varies from person to person and day to day. During meditation some may only concentrate, some may actually meditate and some may go into Samadhi. Most of us wander from concentration to meditation.

Once in meditation or Samadhi, by fixing the mind on various structures, internal or external, one can achieve siddhi powers. For e.g., by concentrating on the tip of nose one can acquire better smelling powers; by concentrating on the tip of the tongue, one can acquire supranormal tasting powers; by concentrating on the middle part of the tongue, one can acquire supranormal powers of touch; by concentrating on the root of the tongue, one can acquire supranormal hearing; and by concentrating on palate one can acquire supranatural colour perceptions (sight). With experience, one can concentrate upon any object of any size, from the atom to the infinity.

Just as pure crystal takes colour from the object nearest to it, the mind when cleared of thoughts achieves identity with the object of concentration.

Primordial sound (beej mantra) meditation is based on the principle that the initial one–point concentration on a particular sound (seed) over a period of time becomes seedless or thoughtless (yoga sutras of Patanjali 1.51).

Swami Vivekananda co-related it with Raj Yoga and said that our average span of attention on a particular object is only around three seconds. He said that if one is able to increase this attention span and concentration at an object of our choice for 12 seconds then we are practicing Patanjali’s sixth stage of yoga or ‘dharana’, which translates as contemplation. And if we can further increase our concentration ability to 12 × 12 seconds or for 144 seconds, then we have reached the mental plateau of meditation or ‘dhayana’. Swami Vivekananda further went on to attribute values to the exalted state of samadhi or transcendental conscious mental state which in value is termed as arising from a meditative or concentration span of 12 × 12 × 12 seconds, which is 30 minutes or half an hour.

Vedanta describes it in terms of units. It says that if you can concentrate 12 seconds on a subject uninterrupted, it becomes one unit of concentration; 12 such units of concentration make one unit of meditation; 12 units of meditation lead to the first stage of samadhi and 12 units of this samadhi lead to the highest samadhi, the supreme realization of Atman. Dharana is concentration; Dhyana is meditation and Samadhi is trance.

Patanjali called them as ‘Matra’: If you are able to sit, withdraw the mind and fix it upon a focal point within (it may be gross, subtle or anything), and are able to keep the mind fixed like that for a period of 12 Matras – a Matra is approximately a moment or a second – it is counted as ‘one concentration’. It says "If you can keep the mind steady without moving, without any contrary thoughts coming in, and without moving away from the object of concentration for a period of twelve Matras, it is regarded as ‘one Dharana or one concentration’". He further says that one should go on practicing this Dharana for days and weeks and months so that it becomes longer and longer. By continuous practice, if one is able to keep the mind focused upon one single point without moving here or there, for 144 seconds (a period of 12 Dharanas), then the person is called Dhyani or a Dhyana Yogi.

Yoga sutra of Patanjali (3.6) clearly says that meditation must be learnt in stages. It calls for repeated practice of meditation. The three basic components of meditation are: The subject of meditation, the center of consciousness at which the mind is held, and the method employed to guide the mind to concentration. The subject of meditation may be the non–dual all–pervading Self, any specific aspect of the divine, or any divine incarnation. The center of consciousness may be at the heart, or between the eyebrows, or at the crown of the head. The method employed to invoke concentration may be any of the following: Japa, or repetition of a sacred word; discrimination between the real and the unreal; dispassion, which is knowing the evil effect of sense–enjoyment; pranayama, or control of breath and ceremonial observances.

But regularity is most important. One can start with looking at any object – flame, idol, or picture for 12 seconds with total concentration and without blinking eyelids (concentration). And then one practices 12 concentrations to make one meditation. The proper meditation thus need not last more than 2 minutes 24 seconds.