User blog comment:Sci100/2014 - A Year of Change: Update 2/@comment-4848837-20140308194139/@comment-2066094-20140308200233

A parallel universe is a different universe that occupies the same timeline. However, the events that transpire in this universe are (usually) independent of all other universes. A different timeline is when something happens in a universe in different ways, at the same time. You can have multiple timelines in 1 universe, but not multiple universes in 1 timeline. That said, there is the (real) theory that every possible action has happened, and that a separate universe is created for each one: While walking, I choose to go left, and another universe is created where I choose to go right. This could be interpreted as both a multiverse and multi-timeline scenario. - StarTrek explanation