The word meditation is sometimes used to refer to the act of reflection or relaxation. But in the context of contemplative traditions meditation involves habituating the mind, training it to cultivate specific mental states, habituating it to think or to visualize in a certain way or simply let the mind observe itself. There are also analytical meditations where you begin by reflecting on a topic but once the understanding is reached, you let the mind dwell in that understanding. So, meditation involves staying or arriving through a method to a non-conceptual experience.
Every day you meditate unconsciously, as you repeat and become used to certain thoughts, moods and reaffirm emotions and behaviors. Some of us are experts in meditating on anger, others on visualizing our enemies, self-criticism, and so on. There are also those who have trained to respond with patience and tranquility in any circumstance. But we all know that it is much easier to cultivate a negative habit than a positive one, because we tend to blame others and justify ourselves. Without realizing it, every day we are reinforcing habits and behaviors that cause us dissatisfaction and suffering.
Meditation is also called contemplation. Since it lead us to a state of contemplative or non-conceptual consciousness. This is not something from other world, we have all experienced it, for example when we are concentrated in an activity that does not require much analysis and reasoning, allowing us to enter a state of flux, we lose notion of time because we are totally present and relaxed. Like when we go out on the road and after some time of monotony in which we relax because there is not much to do, the driving is done almost automatically and there are few changes, we enter that state of relaxation that at the same time is very alert, but it doesn't require a great intellectual process. Some authors speak of the left and right hemispheres, they say that when the left one doesn't have a logical task to solve, it allows the right hemisphere to take control and this allows a more panoramic perception that includes all details and the relationships between them.
In many spiritual, philosophical, and contemplative traditions this ability of the mind to train itself has been valued greatly, and methods for consciously performing this training have been developed.
However, each tradition has given a different flavor to the contemplative practice, some have made it more devotional to produce certain exalted mental states, others have made it more technical for people who need to follow a technique step by step, others have developed analytical meditations to help people who are very rational to come to a conclusion through analysis and then to let go of the analysis while they rest to the mind allowing the acquired knowledge to settle, so to speak.
Thus, an endless number of different forms of meditation have been invented or discovered, with different aims and conditions.