In India, the colloquial word dhyana is commonly translated as meditation. Dhyana also means being attentive or paying attention and that is a very useful meaning for the word "meditation." In that sense, meditation is conscious living.
There is a formal word called dhyana as well. It is one of the eight limbs or steps of Patanjali's Yoga Sutras. The eight limbs are: yama, niyama, asana, pranayama, pratyahara, dharana, dhyana, and samadhi. Yama and niyama are codes of personal and social conduct. Asana (conscious movement) and pranayama (conscious breath) have been described in separate sections of their own. Pratyahara is turning mental awareness from sense objects and sensory experiences associated with them to the internal space within. Dharana is focus or concentration. This focus can be internal or external. Dhyana has already been explained here and samadhi is enlightenment.
According to Mircae Eliade in Yoga: Immortality and Freedom, in Isvara Gita 12 pranayamas on an object of meditation is dharana or concentration. Twelve pranayamas is 12 controlled, equal, and retarded respirations. So 12 pranayamas = 1 dharana. 12 sustained dharanas on a single point = meditation. Meditation = 144 focused pranayamas on the object of meditation. In the structured process of meditation, breath is one step, one unit of meditation. We walk one step at a time, no matter how long the walk. We meditate one breath at a time no matter how long the process. All that is relevant is the one step, the one breath, the one moment. We can simplify it and say that meditation is sustained, one-pointed concentration or focus!
There are so many ways meditation is defined. According to Patanjali, it is the cessation of mental activity. For some, meditation is being in the present moment or mindfulness. What is clearly established is that meditation is extremely useful in learning to relax, deal with stress and anxiety, improving concentration and focus, and knowing your self.
As with asana and pranayama, there are many styles of meditation. Choose a path that fits your needs and do not keep switching. There are infinite ways to reach the Infinite but if we confuse the means with the end, we will circle around lost.
We have introduced some simple meditation practices that anyone can benefit from. For further information, please refer to the article What is Meditation?.