Visualization is the projection of positive images throughout the mind in order to settle the mind, to struggle with disease or psychological shortcomings and stress, or to assist meditation.
But visualization is not meditation. Both visualization and meditation represent effort, but the former is effort that must be consciously, deliberately, even meticulously learned and applied. Of course, meditation must be learned and applied with effort, too. But meditation is an emptying of images, ideas, and thoughts, whereas visualization is a conscious contrivance of thoughts and images for a therapeutic end. In Buddhist tradition, a mental image is the equivalent of a sensory object, as much as is a contrived or entertained thought.
Meditation traditions hold a variety of mantras, but these, too, can be considered visualizations, or auditory visualizations. Zen Buddhism argues against mantras because mantras require a process of mind akin to visualization. Even a “good” mantra is an effort, and potentially a distracting effort. As commentator Nyogen Senzaki explains:
If you drop a coin into a calm pool, the ripples increase one after another. It makes no difference whether the coin is gold or copper. The moment the sea of our mind raises a ripple, the calmness is disturbed and the peace broken.