The appearance of individual pixels and pixel blocks in a still or video image. For example, when a still image is displayed or printed too large, the individual, square pixels are discernible to the naked eye, especially around the edges where one color or shade of gray meets another. When digital video is played, the hardware may not be fast enough to keep up with the frame's encoding rate. Since digital images are compressed in blocks of pixels, one or more blocks may not be fully decompressed in time, which results in visible pixelation. See blocking artifacts
Images are often pixelated on purpose for special effects. For example, pixelation can be used to transition one frame into another, as well as hide nudity and sensitive data or camouflage a person's face. See videophile
The signal from the cable company was momentarily delayed, and the TV was unable to decode the frames fast enough to render them properly.
Pixelate On Purpose
Two Photoshop filters were applied to the top image to create special effects. The Mosaic filter created the left image, while the Crystallize filter created the one on the right. See image filter