A precondition is something that must already be true, or must already have happened, before the next thing can start. Think of a locked door: having the key is a precondition for going in. Countries set preconditions for peace talks; a school may set preconditions for joining a club. Though actually, people also use it as a verb, especially in technical English: to precondition a room is to get the air ready before people arrive; to precondition a battery is to warm or cool it so it works well. That verb is about preparing something in advance, not about a rule. The noun is the one you will meet in news and essays. It sits near requirement and prerequisite. Precondition often sounds a little more official, as if the talks or the deal cannot even begin until this piece is in place.