A broad change to Google's main ranking algorithm, rolled out several times a year and explicitly announced by Google. Core updates re-evaluate the relative quality of every page in the index against current standards; a site that loses ground in one rarely recovers without substantive content quality improvements. Google's standing advice is: there's nothing specific to "fix" — focus on the questions in their core updates guidance about content quality, expertise, and presentation. Recoveries typically require waiting for the next core update to land, which can be months.