refers to an image where like colors in the subject and scene do not change abruptly; the opposite of posterization.

a photographic image represented by smooth graduated tones from one color to another. Continuous tone images must be converted into bitmap files before they can be reproduced on digital devices such as a computer.

The smooth infinite gradation of grays from black to white with out banding. This gradation also produces the full range in colors. (photographic quality printing)