The term "failbreak" is used to refer to a jailbreak that is incomplete - features are missing that would allow it to receive the full functionality users expect, such as tweak injection. Some "failbreaks" depend on Apple binaries that cannot be downloaded separately - for example, parts of Xcode that require an Apple Developer account to download. Some people also call fake jailbreaks "failbreaks". Since this word has multiple possible meanings, it's confusing to try to use it in conversation, so it's best to avoid it.
saurik has said that "the term was actually first used years ago by chpwn on a released jailbreak as there was something wrong with it that caused Substrate to only work in some processes; I was then later using it with regards to jailbreaks where the kernel patches didn't support the various memory protection changes required by C Substrate. I provide a tool called 'vmcheck' that people developing jailbreaks use to 'unit test' their patches, and when it fails... well, that's a 'failbreak'."
This term came up again when chpwn showed a screenshot of his new iPhone 5 running Cydia, shortly after the iPhone's release on 19 September 2012. He explained this as "the “failbreak” is for jailbreak developers (e.g. @iphone_dev, @chronicdevteam, etc)." On 19 October 2012, planetbeing tweeted that he upgraded the "failbreak" with a kernel exploit so that tweaks actually work on the iPhone 5, to make it "almost a full tethered jailbreak."
List of Failbreaks
- iOS 4.2.1 (Jailbreak Monte)
- Apple Developer account required to download iOS 4.2 beta 3
- iOS 6.x on all A5 and A6 models (amfi_interpose)