Short for 'Invision Power Board', which, as of writing, has recently leveled out it's version to 2.0.3.
Invision Power Board, or IPB, was created originally with the acronym IBF, and was then renamed to IPB later on. The first releases of IPB started at 1.0, and later on versions 1.1, 1.2, and then 1.3 appeared, before the step up to 2.0.
With the release of 1.3.1, the license was changed so that 'continued usage' of IPB required the purchase of a licence, and that it was a 'trial only'. However, the software included practically no limitations or lack of functionality on the trial (except for support), and the lack of specifications on 'continued usage' meant the 'trial' was vastly used for production use.
Shortly after the release of IPB 2.0, IPS closed their forums for a few hours, and, after reopening, the trial was no longer available for download. In an attempt to cover all evidence of their previous promise that there would always be a free version of IPB available, they wiped the off-topic forums and silently edited old announcements regarding 1.0 and 1.1.
These changes brought a semi-riot for a while, and IPS shortly after released a Zend
encoded version of the trial with built-in limits, as a replacement for the original trial. IPS had previously discontinued support for those without a licence, and so exists the infamous 'We do not offer support for IPB nor allow it on our forums' message;