In extremely simple terms, the places that you go to where you cross a portal. The worlds that could have been.

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.