Computer manipulation of images. Some of the many algorithms used in image processing include convolution (on which many others are based), FFT, DCT, thinning (or skeletonisation), edge detection and contrast enhancement. These are usually implemented in software but may also use special purpose hardware for speed.
Image processing contrasts with computer graphics, which is usually more concerned with the generation of artificial images, and visualisation, which attempts to understand (real-world) data by displaying it as an artificial image (e.g. a graph). Image processing is used in image recognition and computer vision.