Apple originally used the name iOS for the operating system that ran on iPhones, iPads, and iPod touches.
Starting with iOS 13 and iPadOS 13, Apple started using different names for the iPhone OS (iOS) and the iPad OS (iPadOS). These operating systems still seem be very much in sync with each other with respect to features, app compatibility, and version numbering. However, the iPadOS name is apparently intended to make Apple's naming scheme more regular and to emphasize that there may be some divergence in iPhone and iPad features.
These days, Apple likes operating system names of the form fooOS, e.g.
- iOS for the iPhone OS. (Note: "IOS" with a capital "I" is the name of an operating system for Cisco routers.)
- iPadOS for the iPad OS
- macOS for the Macintosh OS
- tvOS for the Apple TV set-top box OS
- visionOS for the Apple Vision goggle OS
- watchOS for the Apple Watch OS