"Earth-Sigma" is the combined Earth resulting from the disruptions caused by the Crisis on Infinite Earths, incorporating the combined histories of Earth-1, Earth-2, Earth-4, Earth-S, and Earth-X. Many found that this merger changed their histories or erased their pasts from existence entirely. In general, Earth-1's history dominated while contradictions from the other Earths were erased.
A final wave of chronal energies, which rewrote people's memories and initiated the last few changes to match the new timeline, was held back by Mekanique until her mission was completed.
All of the specific changes are too numerous to list here in full, and can be found in the characters and places' individual entries. The designation "Earth-Sigma," with Sigma meaning "sum of all that came before," was coined by Jonathan Woodward of the Annotated Crisis site.