A patch is an update to software to fix any issues; occaissionally new features may also be added. Here you'll find all of CK2's patches, whether they are official or unnofficial.
“Patch 1.06 was released on the 26th June alongside three related DLC's. As well as the usual bug...1.06
“Faction System, Characters can have both a Plot and an Ambition, Retinue System support for the...1.07
“Patch 1.10 was released on the 28th May 2013. Completely revised technology system, Revised...1.10
“Hotfix patch 2.0.4 was released on the 17th January 2014. Fixed a serious issue where big cloud...2.0.4
“Patch 1.111 was released on the 13th August 2013. Fixed the delayed event bloating of save...1.111
“Patch 1.06b was released on the 4th July 2012. The date widget arrows in the lobby no longer...1.06b
“Fixed an issue with broken portraits and other graphics on certain older graphics cards, Fixed a...1.07b