When DC Comics first introduced the idea of a multiverse, the main line of comics being published by the company took place on a world conveniently known as Earh-1. Since then, though, the "main" Earth has changed names a number of times, and in the currently-ongoing Dark Nights: Metal event, it appears as though the post-Rebirth status quo is now called "Earth-0."
Earth-1, in its traditional form, ceased to exist when the Crisis on Infinite Earths took the...well, infinite earths...of DC's multiverse and condensed them into one world, using primarily elements of five popular worlds populated by DC characters, and characters they had acquired from other publishers.
The resulting Earth did not get a formal designation in the multiverse for a long time -- mostly because there was no multiverse after Crisis. Eventually it would be split apart, re-merged, and re-christened New Earth in the sequel, Infinite Crisis.
The "New Earth" designation stuck until 2011's The New 52 reboot, when the main publishing universe started being called "Prime Earth" by fans and on the DC Wiki to differentiate it from the post-Infinite Crisis, pre-Flashpoint world.
While the changes made to DC's timeline following the events of last May's DC Universe: Rebirth one-shot feel like they re-created the world, there has been no confirmation yet that the world itself is demonstrably different from the world on which the New 52 stories were told -- except for Superman, who has a special circumstance.
In Batman: The Red Death, one of a number of one-shots featuring the stories of the "Dark Knights" who are battling DC heroes in Metal, most of the story takes place in the Dark Multiverse, but an establishing caption that brings the action back to the main DCU refers to the world as "Earth-0," the first world in the multiverse.
We can find no record of post-Flashpoint, pre-Rebirth comics referringto the world as "Earth-0," so it is possible that Metal is introducing the concept. Equally, it could be that a tie-in issue in a story like Convergence or The New 52: Futures End, both of which dealt with DC's multiverse, could have established it and we missed it in the torrent of tie-ins.
In a way, it makes sense. Ideas like "New Earth" and "Prime Earth" do not fit into the numbering scheme of DC's multiverse and, particularly with "Prime Earth," the publisher runs the risk of readers confusing it with "Earth Prime," a world in which there are no superheroes except the ones in comic books (read: our world).
Earth-One is also taken at present, with a line of original graphic novels DC is creating aimed at making their superheroes accessible to casual viewers who might try to check out a comic at their local Barnes & Noble after a screening of the Wonder Woman movie.
Of course, there is still a lot of Metal left, and then the universe-hopping Doomsday Clock coming in November. There is still time to confuse us all with another change...!