A pixel is the smallest unit of a digital image. The word comes from "picture element". A digital photograph is a grid of many pixels, often millions, and each pixel holds one color and brightness value.
Seen from a normal distance, the pixels blend together into a smooth image, a bit like the small stones in a mosaic. If you zoom in very far in an editing program such as Photoshop, you can see the separate square pixels and even change them one by one. A camera's resolution is usually given in megapixels, which means millions of pixels.
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