What is upon definition?

The word "upon" is a preposition that indicates a close understanding or awareness of something. It is often used to describe a specific location or time in relation to an event or action. For example, "upon arriving at the train station" or "upon completion of the project." The term "upon" can also be used to mean "on top of" or "above," such as in the phrase "upon the hill." Additionally, it can be used to imply a sense of immediacy or urgency, such as in the phrase "upon receiving the news."