A parallel universe is a hypothetical self-contained reality co-existing with one's own.
Reality is the state of things as they actually exist, rather than as they may appear or might be imagined.
The Universe is all of time and space and its contents.
Infinite Worlds: A Journey through Parallel Universes by World Science Festival
A specific group of parallel universes are called a "multiverse", although this term can also be used to describe the possible parallel universes that constitute reality.
The multiverse is the hypothetical set of possible universes, including the universe in which we live.
Scientists May Have Just Discovered a Parallel Universe by Beyond Science
While the terms "parallel universe" and "alternative reality" are generally synonymous and can be used interchangeably in most cases, there is sometimes an additional connotation implied with the term "alternative reality" that implies that the reality is a variant of our own.
A synonym is a word or phrase that means exactly or nearly the same as another word or phrase in the same language.
The term "parallel universe" is more general, without any connotations implying a relationship, or lack of relationship, with our own universe.
A universe where the very laws of nature are different – for example, one in which there are no Laws of Motion – would in general count as a parallel universe but not an alternative reality and a concept between both fantasy world and earth.
Fantasy is a fiction genre that uses magic or other supernatural elements as a main plot element, theme, or setting.
A fantasy world is a fictional universe created in fiction media, such as literature, film or games.
The actual quantum-mechanical hypothesis of parallel universes is "universes that are separated from each other by a single quantum event."