If something is prearranged, people already planned it, and agreed it, before the moment you are talking about. The meeting time, the seats, the price, the story you will tell — all of that can be prearranged. It is a quiet, ordinary word, more at home in plans and official notes than in excited talk. Though actually, it often carries a small extra feeling: not just “we planned it,” but “it was settled in advance, so nobody is making it up on the spot.” A prearranged signal is one you agreed on earlier; a prearranged visit is one that was booked, not a surprise. You will also see it as the past form of the verb prearrange: “They had prearranged the whole day.” The opposite feeling is something spontaneous, or decided at the last minute.