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 1.06b was released on the 4th July 2012. The date widget arrows in the lobby no longer...1.06b
“Patch 2.0 was released on the 18th November 2013. Ironman mode, Steam Achievements, Religious...2.0
“Patch 1.06 was released on the 26th June alongside three related DLC's. As well as the usual bug...1.06
“Hotfix patch 2.0.3 was released on the 20th December 2013. Fixed crash when stepping around in...2.0.3
“Hotfix patch 2.0.4 was released on the 17th January 2014. Fixed a serious issue where big cloud...2.0.4
“Faction System, Characters can have both a Plot and an Ambition, Retinue System support for the...1.07
“Fixed an issue with broken portraits and other graphics on certain older graphics cards, Fixed a...1.07b