An intercom is a small talking system that lets people speak to each other from different rooms, floors, or parts of a plane, without using a normal outside phone. In Taiwan apartment buildings, the box by the door that lets you talk to someone downstairs is an intercom. Offices, factories, and aircraft use them too. You can also use it as a verb: “I’ll intercom you” means “I’ll call you on that inside system.” It is everyday and a little technical, not fancy. Though actually, people sometimes call any inside building phone an intercom, even when it is just a normal inside line. It is not the internet, and it is not a mobile phone.