What's the difference?

[DonIago]

I thought those were alternate universes, not timelines.

I'm not convinced that there's a difference.

 

[BillJ]

I'm not convinced that there's a difference.

I tend to agree where Trek is concerned.

 

[Shawnster]

All alternate timelines are alternate dimensions but not all alternate dimension are alternate timelines.

 

[FredH]

For the record, I hate the idea that all alternate timelines still exist after time-travel happens; it removes any stakes in time-travel stories. What was wrong with the Borg changing history in "First Contact" if their new timeline didn't change the old one? And what would stop a villain from using time travel to make a clone army? Sorry for the rant, I know you didn't ask.

I personally think the only way to square it is to assume that, due to higher-dimensional physical laws beyond our ken, in some situations it creates an additional timeline, but in other situations it erases and changes a single timeline, and there’s basically no way to predict which it’s going to be.

I’d also say that, whatever the technical definitions, in practice “alternate timelines” and “alternate universes” seem to get used as different names for the same phenomena: realities that are different from each other, for whatever reason.

 

[Frontis]

In an alternate universe you usually meet the evil versions of the characters
In an alternate timeline you take characters and other elements form a story, don't care about what happened before and write your new own story.