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 1.103 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
“Fixed an issue with broken portraits and other graphics on certain older graphics cards, Fixed a...1.07b
“Patch 1.103 was released on the 4th July 2013. Fixed incorrect AI reasoning text string "X is my...1.103
“Hotfix patch 2.0.4 was released on the 17th January 2014. Fixed a serious issue where big cloud...2.0.4
“Patch 1.09 was released on the 14th January 2013. Merchant Republic gameplay (Duke or higher...1.09