iOS 11 is the eleventh major release of iOS, released on September 19, 2017, with the iPhone 8 generation and — six weeks later — the iPhone X, whose gesture interface, Face ID, and notch-aware layouts it carried. It was two releases in one: a transformative iPad update (a Mac-style Dock, a true app switcher, drag and drop, and the Files app finally exposing a filesystem) and, on iPhone, the platform's rockiest quality cycle — a cadence of bugs, battery complaints, and emergency point releases that pushed Apple into the stability-focused iOS 12.

It also closed the 32-bit era: apps never updated for 64-bit simply stopped launching, cutting off a museum's worth of early App Store history — and it hosted the battery-throttling controversy, whose December 2017 disclosure produced Apple's apology, $29 battery replacements, and the Battery Health controls delivered in 11.3.

ARKit — instant, calibration-free augmented reality for hundreds of millions of devices, the largest AR platform launch ever — headlined for developers. Users got a customizable, unified Control Center; screen recording; Do Not Disturb While Driving; a one-handed keyboard; person-to-person Apple Pay (11.2); a redesigned App Store with editorial Today content; and HEIF/HEVC media formats halving photo and video sizes. The iPad's Dock, Split View via drag, Files, and Apple Pencil integrations effectively previewed iPadOS.

iPhone X, iPhone 8/8 Plus back through iPhone 5s, iPad Air/mini 2 and later, and the sixth-generation iPod touch — dropping the 32-bit iPhone 5 and iPhone 5c.

ARKit seeded the spatial-computing lineage that surfaced in visionOS; the Files app and iPad Dock became iPadOS's foundation; and the X's gesture grammar, shipped inside this release, became the platform's permanent interaction model. But iOS 11 is equally remembered as the quality low-water mark that forced a course correction — the reason "this year focuses on performance and reliability" became a recurring WWDC sentence, starting with its successor.