Someone who watches. The word is much less common on its own than you might expect — English speakers usually say 'someone who watches' or use a more specific word. Where 'watcher' really lives is at the end of a compound, naming a hobby or a job: a birdwatcher, a whale watcher, a clock watcher (someone waiting for work to end), a Weight Watcher. In news and business English it names a person who follows one subject closely and is expected to explain it: 'China watchers', 'market watchers', 'Fed watchers'. These people are half expert, half professional noticer. There is also a colder use, of someone watching to guard or to keep an eye on others — a night watcher, or in stories, silent watchers in the dark. That use can feel a bit frightening, because a watcher does nothing but look. And note the plain, warm use too: 'She's a people watcher' just means she enjoys sitting and looking at people go by. What the word never quite means is 'viewer'. For TV or films, English says 'viewers' or 'the audience'.