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 2.0 was released on the 18th November 2013. Ironman mode, Steam Achievements, Religious...2.0
“Patch 1.06b was released on the 4th July 2012. The date widget arrows in the lobby no longer...1.06b
“Faction System, Characters can have both a Plot and an Ambition, Retinue System support for the...1.07
“Hotfix patch 2.0.3 was released on the 20th December 2013. Fixed crash when stepping around in...2.0.3
“Patch 1.06 was released on the 26th June alongside three related DLC's. As well as the usual bug...1.06
“Fixed an issue with broken portraits and other graphics on certain older graphics cards, Fixed a...1.07b
“Patch 1.09 was released on the 14th January 2013. Merchant Republic gameplay (Duke or higher...1.09