Meditation is not concentration. Concentration is holding the mind to something within or outside the body. On the other hand, 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.
When the mind can continue in that concentration for 12 times (12 seconds x 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 x 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.
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 twelve 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 twelve Dharanas), then the person is called Dhyani or a Dhyana Yogi.