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human
[MCU] What difference between universes, timelines, and realities?

Venom and Loki spoilers will be basis for the background of my question. 


Loki and the TVA suggest that different timelines exist that branch off the “sacred timeline”. Which would be basically the MCU. MCU between Dr. Strange, Infinity War, End Game and Loki suggests that timelines are different realities in which choices affected outcomes of great importance. However in all these timelines every character that we know from the MCU exists. The characters don’t always become what they are meant to be but they still exists. 

They use the words universe and timeline interchangeably and it doesn’t really make sense to me. To me a different universe would be one where most if any of the MCU characters don’t even exist. A different timeline would be if Tony Stark was just a regular mechanic. 

Loki supports my belief I feel because there’s never any evidence that any of the other universes existed ie X-men or Amazing Spider-Man. There’s variants of TVA, of Loki, or Kang but never any of Wolverine or evidence that Adamantium exists. 

All this to say I just watched the new Venom and the post credit scene while enjoyable makes no sense. 

How is it possible that Venom can now exists in the MCU? Surely if it was a branch of the “sacred timeline” at least some elements of the MCU would have appeared before now.
gpt
A universe or reality is a single dimension that normally contains an alternate Earth. The MCU/Earth-19999/Reality-19999 (Avengers films) and Earth-616/Reality-19999 (comics universe) are examples of universes. The "real world" universe (our own) has been called Reality-1218. They are also called parallel universes, in that they co-exist with other universes with no direct interaction, such as the MCU and Earth-616 are similar but no derived from the same source. Picture two rivers flowing parallel but neve overlapping.

A timeline is often used as another term to describe other universes or realities, but more specifically it is used when referring to that universe when time travel is involved. Whenever one time travels, they normally cannot enter their own timeline (ie they cannot interact with their direct past) but rather a divergent timeline branches off from the main timeline, effectively identical other than the chances made by the time travelers. Divergent timelines can also occur on their own without time travel. Picture on river that branches into a second stream, the first stream remains unchanged but their is a second one flowing from the same source.

The term alternate universe is a blanket term to describe all variations of universes or realities.

The Multiverse is the collection of ll the realities and timelines of the Marvel continuity (all the comics, films, TV series, etc). In a broader context, there are also other Multiverses, each one containing their own franchise. There is the DC Multiverse, the Star Trek Multiverse, the Doctor Who Multiverse, etc. In general, characters from one multiverse do not interact with characters from another unless there is some intercompany crossover (like the comic Justice League/Avengers).

There is also the Omniverse, a term effectively describing all the universes and multiverses as a whole.