Timelines are the different alternate universes that have been created over the span of time. Although they have different names, the most common name for them is "Multiverse", or "Alternate Universes". They may also refer to the "Many-worlds interpretation".
"The multiverse is the hypothetical set of finite and infinite possible universes, including the universe we live in. Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, and the physical laws and constants that describe them."
-Definition of Multiverse, Wikipedia.
"Many-worlds implies that all possible alternate histories and futures are real, each representing an actual "world" (or "universe"). In lay terms, the hypothesis states there is a very large—perhaps infinite—number of universes, and everything that could possibly have happened in our past, but did not, has occurred in the past of some other universe or universes."
Definition of the Many-Worlds Interpretation, Wikipedia.
Actual ones are:
1- The main MIRW universe, but now impossible to use as the universe was destroyed here.
2 - A Dystopian version of the first and the fifth.
3 - A version where siberiat crushed and fused with gaia.
4 - An inverse of the first and the fifth.
5 - Actual RP Goes on in there because it's a remake of the first one.
6 - The original, MI Forums Chat one. It is the one that started it all but eventually split into multiple timelines. It is where most OC's originated, although it didn't last too long.
7 - Insanity Ward, loosely formed by what used to be early MI Forum Chat Roleplayers and is based mostly around the MI Forums timeline.
8 - Manceri, The timeline that the original Mancers came from, actually, it is now pure darkness and uninhabitable.
1- Only a person with extremely advanced time-based technology or god-like beings with advanced time manipulation abilities are able to travel through different timelines.
2- The person who moves between timelines has a somewhat low chance of vanishing and shattering into timelines.
3- If you move something really important from a timeline to another, it will be erased.
4- Erasing a timeline is near impossible, If you want to do it, do it on a timeline with different physics that doesn't apply to the canon timeline.
5- Timelines may branch off into different timelines depending on different choices.