CCTV is the written short form of closed-circuit television: cameras that send pictures to a small set of screens, not to every television in the country. Shops, stations, schools, and streets use CCTV so staff or the police can watch what is happening. “Closed circuit” is the old technical idea — the signal goes round a private loop, not out as a broadcast. Today the pictures often travel over computer networks, but people still say CCTV for those security cameras. You see the letters on warning signs, especially in Britain (“CCTV in operation”). Americans more often just say “security cameras.” In speech, Taiwanese students usually say 監視器, even when a sign writes CCTV. One big trap: in Chinese, CCTV is also the usual English short name for China Central Television (央視). Same letters, completely different thing.