In extremely simple terms, the places that you go to where you cross a portal.

Everything that can happen, happens; just not in the same place. In fact, what happens determines the place. In this context a timeline could be considered a sequence of interconnected moments that start in the universal big bang and end at the heat death of the universe, but this is merely the way we humans look at it.

Because a moment in time doesn't have just one past or one future. For every different choice that can be made, there is one different future moment splitting from this one, and for every different detail that can be forgotten, there is one different past that resulted in this moment.

For every different choice and every different detail in the whole universe. A choice made or a detail forgotten twenty galaxies away will split everything the same as if it happened right here.

Forget everything related to anything being unique. Nothing is unique in the entirety of the multiverse. Everything repeats a lot.

It is impossible and useless to refer to a specific moment in time, or to the full line that goes from the beginning to the end, instead we refer to a bunch of possible similar sequences that are similar enough as a time line (even though the name would be time segment).

Or if you speak Shon you can call them Lazankú.

Sometimes, when talking about a wider, rougher set of moments in time, such as the set of all lines where the dominant human species in earth is homo sapiens, we use the term time branch.

It is technically possible to go even further beyond to places that do not even have the same Big Bang in common with our particular timeline. These places can be weird, as in, the laws of physics themselves might not be the same.

And because of this, even if you could spare the ludicrous amounts of energy required to reach these places, they would not be compatible with your physics and you would instantly cease to be the moment you tried to step through a portal. If the interaction between the two competing multiverses does not make the portal into something that erases both timelines first.

The few beings that have managed to cross eternity in that way have had to be “translated” into the other multiverse in some way that's beyond the capabilities of any known human civilization.

Dreamworlds are not usually separate timelines; they are more like the Matrix: a shared illusion built across sentient minds in a particular timeline.