means A person who watches something — a screen, a show, a stream — or a device or window through which something is seen.

from From 'view,' which came through Old French 'veue' (a sight, a seeing) from Latin 'videre,' to see — the same deep root that gives us 'vision,' 'video,' and 'evident.' Tack on the agent-suffix '-er' (the one who does the thing) and you get 'viewer': literally 'one who sees.' The word sharpened into its modern screen-bound sense in the 20th century, riding the rise of film and television, when the seer quietly became the watched-over audience.