Multiverse

The Multiverse is the term for the collective parallel worlds that various versions of Larry Laffer has traveled to during his various adventures, or reliving events of Leisure Suit Larry 1.

Background
The initial trip across the Multiverse or time warp takes place in LSL3 when Larry and Patti use a magic marker to reach real world and land at Sierra Studios, and become the designers of his own games, thus insuring that he existed.

Leisure Suit Larry VGA, Leisure Suit Larry Reloaded exist in one of Larry's parallel universe worlds, and Wet Dreams Don't Dry may also exist because of a paradox across the 'multiverse' as well.

Behind the scenes
The the concept of the 'multiverse' is established in LSL3, but is first mentioned by name in the Bedside Companion books. Which expands and explains the events of his travel between worlds in LSL3, and his later disappearance during events of LSL4, and his return to his own version of the world (though he later returns to work for Sierra during making of LSL6, 7, etc, see The Official Book Of Leisure Suit Larry).

A similar concept of multiverse or time and space warp appears in the The King's Quest Companion, and The Space Quest Companion as well, both by Peter Spear.