Short answers, pulled from the story.
The isekai genre originated from the ancient Japanese folktale of Urashima Tarō, which established the foundational template for a person transported to a different realm. This story dates back centuries and created a cultural ancestor to modern narratives where protagonists find themselves stranded in worlds that operate on entirely different rules of time and existence.
The first modern Japanese work to explicitly center on a character transported to another world was Haruka Takachiho's novel Warrior from Another World, published in 1979. This novel, along with Yoshiyuki Tomino's television series Aura Battler Dunbine in 1983, marked the beginning of a new era in Japanese fiction where the protagonist's journey into a foreign reality became the central plot driver.
The true explosion of the isekai genre began in the late 2000s with the rise of the website Shōsetsuka ni Narō, which translates to Let's Become Novelists. This platform allowed amateur writers to publish their stories online, leading to a surge in isekai-themed web novels and the creation of original isekai novels that would define the genre for the next decade.
A defining meme of the isekai genre is the truck, a vehicle that appears in countless stories to kill the protagonist and send them to another world. This trope, known as Truck-kun, has become so ubiquitous that it is now a standard method of death for characters in reincarnation isekai.
The word isekai was added to the Oxford English Dictionary in March 2024. This addition marked the genre's transition from niche web fiction to a dominant force in global entertainment and confirmed its cultural impact.