Meditation is the act of calming and focussing the mind while avoiding rationalistic thought processes and random fantasies.
Proponents of meditation make various claims that it can lead to reduced stress and hostility.
In the samadhi or shamatha forms of meditation, the mind is kept closely focussed on a particular word, image, person, or idea. This form of meditation is found in the Hindu traditions including Yoga and modern Transcendental Meditation, in Medieval Christianity, and in some modern metaphysical schools.
In vipassana meditation the mind is trained to notice each perception or thought that passes, but without "stopping" on any one. This is the characteristic form of meditation in Buddhism, especially Zen Buddhism.