The word "until" is a preposition that indicates a point in time when an action or event will stop or change. It is often used to show that something will continue to happen up to a specific moment. For example, "I will wait until 5 PM" means the waiting will stop at that time. In addition to its use in time expressions, "until" can also imply a condition that must be met before something else occurs. For instance, "You cannot leave until you finish your homework" means that finishing the homework is necessary before leaving.