A parallel universe refers to a universe that has the same basic point as this universe, but has changed at a certain point in time, so it is different from this universe.

The parallel universe and this universe are on different time and space lines.

The parallel universe hypothesis comes from Schrodinger's cat conjecture reasoning of quantum mechanics and is put forward to eliminate the role of observers. The theory is that when the blank line is opened at that time, any different choice of any universe may produce a new parallel universe.

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The multiverse is another matter. Our universe is in a nothingness/quantum sea (a very unscientific but occasionally necessary concept). There may be not only one universe in nothingness, but also many universes. These universes are called multiverses.

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That is, parallel universes exist on different time and space lines (note that they are not time lines).

Multiple universes exist in different spaces or different dimensions.