precondition is something which must be true before some code executes postcondition is something which must be true after some code executes invariant is bot

precondition is something which must be true before some code executes postcondition is something which must be true after some code executes invariant is both: something which must be true both before and after some code executes. So it's the other way around. An invariant is a postcondition, but a postcondition is not necessarily an invariant. (you can check all three with assertions, but they will generally exist whether you check them or not)