Earth-1

Earth One is a universe parallel to a plethora of others, such as Earth Two. The name was coined by Martin Stein. To Earth Two citizens Earth One is referred to as Earth Two. Also, to Earth Seventeen's citizens, it is known as Terra Prime.

Earth One is particularly important in that it is central in the multiverse, being the middle point from which you can travel to any other alternate Earth.

History
After learning of the existence of parallel worlds, Martin Stein coined the world's name, "Earth One", to describe Team Flash's relative position in regards to all of the other worlds.

From Earth Two

 * Laurel Lance/Black Siren
 * Shay Lamden/King Shark
 * Linda Park/Dr. Light

Formerly

 * Jesse Quick
 * Dante Ramon/Rupture (deceased)
 * Al Rothstein/Atom-Smasher (deceased)
 * Eddie Slick/Sand Demon (deceased)
 * Dr. Harrison Wells
 * Hunter Zoloman/Zoom (in the Speed Force)

Formerly

 * Jay Garrick/The Flash

From Earth Nineteen

 * Harrison "HR" Wells

Arrow
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Season 5

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The Flash
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Season 3

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Vixen
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Season 2
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DC's Legends of Tomorrow
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Season 2

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Constantine
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Season 1
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Arrow: The Dark Archer

 * "Sacrifice"
 * "Carnage"
 * "Deception"
 * "Confession"
 * "Tempest"
 * "Blessed"
 * "Traitor"
 * "Betrayal"
 * "Sacrifice"
 * "Vengeance"

Behind the scenes

 * The universe's name is spelled Earth-1 in the synopses for The Flash episodes.