Simultaneity refers to the occurrence of events at the same time. In physics, it is often discussed in the context of Einstein's theory of relativity, which shows that time can be perceived differently depending on the observer's frame of reference. For example, two events that seem simultaneous to one observer may not appear so to another moving at a different speed. In everyday life, simultaneity can be observed in activities like watching a movie while eating popcorn. Both actions happen concurrently, but they can be experienced differently by individuals based on their focus and engagement with each activity.