Reboots, Reality Shifts, and Retcons
In the 1980s DC Comics felt they had a problem. If you were new to comics you could pick up two comics with Superman in them and be unaware they took place in different universes. All the DC Heroes of the 1930s, 40s, and early 50s were from a universe called Earth-2. More modern heroes lived in a universe called Earth-1. Both Supermen had a secret identity Clark Kent, though the Superman of Earth-2 began his career in the 1930s while the Superman of Earth-1 began his career "a few years ago". Similarly the Flash of Earth-2 had as a secret identity Jay Garrick, also beginning his career in the 30s/40s while the Flash of Earth-1 was Barry Allen, beginning his career "a few years ago". Sometimes characters from these universes met each other. Add to these universes ones where heroes and villains swapped, universes for characters from companies DC Comics had acquired, etc. and DC felt their continuity had become convoluted. Their solution was Crisis on Infinite Eart