1) Artistic technique 
Posterization of an image entails conversion of a continuous gradation of tone to several regions of fewer tones, with abrupt changes from one tone to another. This was originally done with photographic processes to create posters. It can now be done photographically or with digital image processing, and may be deliberate or may be an unintended a...
A photographic or digital graphic effect arising from reducing the number of continuous tones in an image. Can be a deliberate effect or a result of over-manipulation or compression in a digital image
A photographic or digital graphic effect arising from reducing the number of continuous tones in an image. Can be a deliberate effect or a result of over-manipulation or compression in a digital image. 
The conversion of an image to a more elementary form by reducing the number of tonal values, creating a surrealistic, stark result.