Prearrangement is the noun: the act of planning something in advance, or the plan that was already agreed. If two friends have a prearrangement to meet at six, the meeting is not an accident; they settled it earlier. You hear it in careful, slightly formal English — letters, contracts, funeral or travel plans — more than in chat. Though actually, many people just say “arrangement” or “we already planned it,” and keep this longer word for when they want to stress that everything was fixed before. It is countable when you mean one such plan (“by prearrangement”), and it can also name the whole business of arranging ahead. Do not treat it as an adjective; that job belongs to prearranged.