iOS 8 is the eighth major release of iOS, released on September 17, 2014, with the iPhone 6 and iPhone 6 Plus. Apple pitched it as "the biggest release since the App Store," and structurally that held: Extensions opened the system's sealed borders, letting third-party apps provide keyboards, share sheets, Today widgets, photo filters, and Safari content blockers — the platform's most significant loosening since native apps arrived.

It also laid rails Apple would build on for a decade: HealthKit and the Health app, HomeKit, Handoff and the Continuity bridge to the Mac, Family Sharing, iCloud Drive, and — via the 8.1 update — Apple Pay.

Extensions and third-party keyboards led; interactive notifications, QuickType predictive text, Continuity (Handoff, SMS relay, iPhone calls on Mac/iPad), Family Sharing, iCloud Photo Library (beta), time-lapse video, and the Tips app filled the launch. iOS 8.1 switched on Apple Pay; 8.4 delivered Apple Music and Beats 1 in June 2015.

The cycle's stumbles were memorable: the 8.0.1 update briefly killed cellular service and Touch ID on iPhone 6 units (pulled within hours), and the ~5 GB free-space requirement to install over the air left 16 GB device owners deleting their photos — a storage squeeze that drew a class-action suit and sharpened criticism of the 16 GB tier.

iPhone 6, iPhone 6 Plus, iPhone 5s, iPhone 5c, iPhone 5, and iPhone 4s (sluggishly), plus iPad 2 and later and the fifth-generation iPod touch — dropping only the iPhone 4.

Extensions rewired what "an app" could be on iOS and remain the mechanism behind every keyboard, widget, and share target since. HealthKit and HomeKit grew into platform pillars; Continuity became the connective tissue of the Apple ecosystem strategy; and Apple Pay, delivered mid-cycle, outgrew them all. iOS 8's bumpy quality record, together with iOS 7's, also set up the pendulum swing that made stability-focused iOS 9 and iOS 12 a recurring Apple genre.