means A network of video cameras that transmit footage to monitors or storage, letting someone (or something) watch a space in real time or review what happened later.

from Closed-circuit television emerged in the 1940s when engineers realized you could run video over a wire loop without broadcasting it publicly—unlike regular TV. The British adopted it obsessively in the 1990s, turning CCTV into a cultural fixture and spawning the phrase 'a camera on every corner.'