The word pixel entered print through pictures from Moon and Mars probes
In 1965 Frederic Billingsley of the Jet Propulsion Laboratory used the word pixel in print to describe the dots making up scanned images sent back by space probes. He had picked it up from Keith McFarland, an engineer in Palo Alto, who said it was simply in use around 1963 and had no idea who coined it.
The two halves are older. Pix, short for pictures, appeared in headlines in the entertainment paper Variety in 1932, meaning films, and by 1938 photojournalists used it for still photographs. The el stands for element, a pattern later copied in voxel for a volume element and texel for a texture element. The idea itself goes back further still: Paul Nipkow's 1888 German television patent spoke of a Bildpunkt, a picture point, and English patents mentioned picture elements from 1911. Engineers at IBM preferred the shorter pel, used in the manual for the original PC. Pixilation, with a second i, is unrelated; it comes from an old British word for being possessed by pixies and names a stop-motion technique using live actors.
A pixel is the smallest piece of a grid-based image that can be controlled on its own, but what that means shifts with context. On a screen, each colour pixel usually holds separate red, green and blue subpixels, which on an LCD are thin vertical strips. Clever text rendering exploits those strips to make letters look sharper than the grid should allow. In cameras, most sensors sit behind a Bayer filter, so each sensing spot records just one colour and the full picture is reconstructed afterwards.
Colour depth follows simple doubling. One bit per pixel gives on or off; every extra bit doubles the palette, so 24 bits yields 16,777,216 possible colours. The common 16-bit format hands green six bits and red and blue five each, because human eyes notice errors in green most easily. A 32-bit pixel adds eight bits describing transparency.
Pixel counts are often loosely called resolution, and the numbers are easy to confuse. An old 640 by 480 VGA picture holds 307,200 pixels, about 0.3 megapixels. Printer dots per inch are not the same as image pixels per inch, since a printer may lay down several ink droplets for each pixel, so a 600 ppi image might print on a 1,200 dpi machine.
Source: Pixel
