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.
“Hotfix patch 2.0.4 was released on the 17th January 2014. Fixed a serious issue where big cloud...2.0.4
“Patch 2.0 was released on the 18th November 2013. Ironman mode, Steam Achievements, Religious...2.0
“Patch 1.103 was released on the 12th June 2013. Added a "Retract Vassal" interaction, Disallowed...1.101
“Patch 1.06b was released on the 4th July 2012. The date widget arrows in the lobby no longer...1.06b
“Patch 1.103 was released on the 4th July 2013. Fixed incorrect AI reasoning text string "X is my...1.103
“Faction System, Characters can have both a Plot and an Ambition, Retinue System support for the...1.07
“Patch 1.09 was released on the 14th January 2013. Merchant Republic gameplay (Duke or higher...1.09