A pixel is one tiny dot of colour on a screen or in a photo. A gigapixel is a billion of those dots (giga- here means billion, the same prefix as in gigabyte). A gigapixel image has so many dots that you can zoom in again and again and still see new detail — a whole city, a museum wall, every brush mark on a painting. The camera in your pocket is usually measured in megapixels (millions of pixels), not gigapixels. True gigapixel pictures are often stitched together from many shots, or taken with special cameras; they are for showing off detail, not for sending in a chat. People sometimes say gigapixel loosely for any extremely detailed picture. Technical; photographers and computer people use it.