“Isekai” is a Japanese word meaning another (or alternate) world. As a subgenre, it’s roughly synonymous with “portal fantasy.”
Now, some folks make the method the method of conveyance (for example, a magical portal, like a the inside of a wardrobe…) an important factor in both definitions. I don’t. In general, it’s not much more than a MacGuffin. In spite of the name, I don’t believe a literal portal is necessary for portal fantasy, nor must the other world be something other than Earth in an isekai.
What the isekai genre means to me is a character from our world being forced to survive, adapt, and make a life in a new setting very different from our own. The story should focus on this new world being viewed through the eyes of someone from our world, which makes them a reader surrogate. The transported character can therefore describe the wondrous new things in terms we understand, like a monster being the size of a bus, or breaking down the magic system into game-like terms.
If the main character is simply visiting, or hopping from world-to-world without any sense of permanence, or the story is focused on this world rather than the alternate world, then in my mind it’s not really isekai. My own series, Blood Creek Witch, divides the story between the two worlds with slightly more emphasis on our world. So I wouldn’t count it. In the manga and anime “Saving 80,000 Gold in Another World for my Retirement,” the main character’s special ability is being able to teleport between worlds at will. The story focuses on the new world, but it’s an interesting gray area. I consider it more “isekai-adjacent” than a true isekai genre piece–which doesn’t make it any better or worse than a “true” isekai story. Likewise “reverse isekai” is a different subgenre entirely with an inverted point-of-view of looking at our world through the eyes of a stranger.
Now, keeping with this definition, the important thing is that the main character is forced to make the new world their home, at least for a while. This is where the method of conveyance may be important. The commitment is important. The Aincrad arc of Sword Art Online certainly counts, as the bad guy has cut off the players’ methods of returning to the “real world,” and the players are forced to make a life for two years in an artificial fantasy world. Later arcs mix it up, but it isn’t until Alicization that we could really call it a true isekai storyline. In GATE: Thus the JSDF Fought There, there is one physical location to go between worlds, but it is under strict government control, restricting travel. In most resurrection fantasies, the character dies on Earth and has no life to return to. At the end of A Princess of Mars, John Carter is returned to Earth against his will, but having made a life on Barsoom, he spends years trying to find his way back.
Okay… so now we go to the question from the title. What about time travel stories?
Well… it depends.
If the character is thrust into a world significantly different from their own, and must make a life there because there’s no convenient point of return, then it counts in my book, regardless of the MacGuffin reason they arrived there. If a character falls asleep for hundreds of centuries a la Buck Rogers, it’s an isekai time travel story. If they travel to the distant past and must make a life there, at least for a while, then it’s isekai. Time-loop stories like Groundhog Day probably don’t count because the world is very familiar (extremely familiar, after a few loops), so there’s not much ‘sense of wonder’ in exploring a new setting through modern eyes. If they are jumping through multiple time periods through the use of a blue box or something, it is something else as well. Maybe something cool, but not really isekai.
So, whether it’s a reincarnation, portal, time machine, hypersleep, astral projection, act of deity, getting digitized by a laser, it all counts as isekai if it meets these loose requirements.
And yeah, my latest book series, The Vanished, is isekai. It starts with the first book, Queen of Monsters. Aiden finds himself transported to another world and granted (unfortunately for him, somewhat glitchy) special abilities. Even more unfortunately, this is a common enough event that the natives are well-prepared for arrivals of people from Earth and have mastered the art of “spawn camping” – immediately capturing and pressing the people from Earth into service as slaves or cannon-fodder.