Many learned persons use the words 'concentration' or 'contemplation' for the word 'meditation'. Both of these words are inadequate. Let us look at their meanings in a dictionary. Random House College Dictionary gives the meaning of the word 'Concentration' as: exclusive attention to one object; close mental application etc. Again as per the dictionary, the word 'meditate' means to think contemplatively; and meditation means the act of meditating; and also thought, reflection, contemplation etc. 'Contemplate' means to look or view with continuous attention. The word concentration conveys the meaning of 'aykaagrataa' that is intended in our context. In the context of yoga, meditation means going beyond sensual perception and thoughts, indeed going beyond the mind itself. Therefore how can 'meditate' in the context of yoga mean thinking contemplatively!! Obviously, the word contemplation as used does not convey the sense of meditation as annotated in Hindu scriptures. Hence I would not recommend use of 'concentration' or 'contemplation' for the word 'meditation'. Since this is used commonly, it needs an analytical study.

Let us go to an authoritative source of the words concentration and 'meditation' viz. 'Patanjali's Yoga Sutra'. Let us be clear about the word Yoga, which in this context means Samaadhi or Moksha or realization of Aatman, the only reality or 'aykmayva adwiteeyam' or 'Sat-Chit-Aanand'

There are eight steps in the Ashtaang Yoga viz.: practice of yama or restraints, niyama or virtues, aasana or postures, praanaayaama or breath control, pratyahaara or withdrawal, dhaaranaa or concentration, dhyaana or meditation, and samaadhi or unitive awareness. And concentration being the sixth distinct step, there should be no doubt about concentration being a prerequisite of meditation'. However as per Patanjali there are six prerequisites for meditation. In this space age, many yogic authorities often delete the first four steps from being prerequisites. However these four steps though difficult and time consuming certainly help in achieving not only meditation but also Samaadhi. The fifth step 'Pratyaahaar' is definitely a prerequisite for Dhaaranaa or concentration. Pratyaahaar means withdrawal of senses from their objects and turning the mind inward.

'Dhaaranaasu ca yogyaatamanasah': 'And the mind organ must be fit for concentration.'
'Desa-bandhas cittasya dhaaranaa': Concentration of the mind is binding the mind to a place.

'Tatra pratyaya-eka-taanataa dhyaanam': The extension of this binding of the mind to a place is meditation. If one restricts oneself to this mantra only, then one may, and many do, conclude that mere binding of ones mind to a place for long periods is dhyaan or meditation. Concentrating on a single thought, even on the image of a God, is not meditation. This is not an appropriate understanding of meditation. True extension of dhaaranaa is meditation, but the extension is not merely in time but in a new dimension, which is beyond mind. The goal of concentration is meditation, and that of meditation is Samaadhi, which means a state in which the observer, the observed and the observation become one. Yoga and therefore samaadhi really means 'Yogashchittawratti nirodhah'(1.2), that is state of Samaadhi is perfect absence of mind's activities. And dhyaan is 'Dhyaan heyaas tad vrattayah' (2.11) i.e. dhyaan or meditation removes 'vatties' i.e. the activities of mind. Here the purpose of dhyaan is not just extended concentration but cessation of activities of mind. Extended dhyaan is Samaadhi. The same meaning is reiterated unambiguously in first three mantras of 'Vibhooti Paad' :

Concentration : The attention of the mind-stuff is directed in a single stream to a chosen field, without being distracted (3.1).

Meditation : The cognition is entirely in the field of observation which is its own field - that is, when the observer is observed (3.2). It is clear from this that this state is, not merely quantitatively but qualitatively different.

Samaadhi : Observation and the observing intelligence merge as if their own form is abolished and the total consciousness shines as the sole substance or reality, without the divided identity of the observer and the observed (3.3). This sate is also known as 'Kaiwalya'.

Patanjali Yoga Sutra also describes 'Beej Samaadhi' and 'Nirbeej Samaadhi','Nirvikalp Samaadhi' etc. and that, not being the subject under discussion, is not being discussed here. Extended concentration on a single thought is comparatively less difficult than extended thoughtless awareness state of mind i.e. 'dhyaan'. Even the extended state of concentration on a single thought does help in steadying the mind and removing stresses. But it is not 'dhyaan' which is a completely still state of mind without any thought. This state has manifold advantages, apart from being a jumping step for Samaadhi. The six enemies do not disturb such a person. He can make fast advance easily towards a 'desireless state' of mind and remain unperturbed by pleasures and sadness . He can get glimpses of Sat-Chit-Aanand, and advance to be absorbed in 'That'.