In DC Comics, an earth in a paralell uinverse. It's the original home of the Golden Age Superheroes like Jay Garrick The Flash, Alan Scott Sentinel and the JSA. They all now reside one Earth-Only.
Before Crisis on Infinite Earths, Earth-Two was used to keep the Golden Age heroes seperate from the Silver Age heroes.
Not to be confused with Earth2, the Anti-Matter Earth (known as Earth-Four before the Crisis on Infinite Earths)