system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is an alternate universe different from an alternate dimension?

Sorry if it's a dumb question, it popped up during shower.
gpt
In science fiction, where you probably picked the concepts from, there is no difference. Alternative universes, dimensions, timelines, realities et al are not recognized by empirical science, so writers can pick and choose whichever term sounds the coolest for a world kind of like ours but not.

In terms of actual science, "alternate dimension" doesn't even make sense.

Dimensions in physics and math act like coordinates. On a flat map you need just longitude and latitude to define a point, which can be thought of as two dimensions. In a three-dimensional scene you would also need the height, and in reality also the time, "when" to go with the "where."

As such, "alternate dimension" doesn't make sense. Mathematical models can work with more than three spatial dimensions and one temporal dimension, but those hypotheticals aren't really "alternative."

"Alternate universe" makes more sense, but they are not scientifically recognized concepts either. There are some theories which postulate that our universe isn't the only one (which is problematic right off the bat because the "universe" by definition includes everything) but there is no empirical evidence whatsoever indicating that they would be real.