The relativity of simultaneity

The relativity of simultaneity is a concept from Einstein's theory of relativity that states that events that appear to happen at the same time for one observer may not appear simultaneous to another observer in motion relative to the first. For example, if two lightning bolts strike at opposite ends of a train moving quickly, a person on the train may perceive the strikes as occurring at different times, while an observer standing still outside the train sees them as simultaneous. This illustrates how time is not absolute but dependent on the observer's relative motion.