Core Update
A core update is a broad change to Google's main ranking systems, rolled out a few times per year over one to three weeks. It reassesses relevance and quality across all sites, so rankings can shift substantially without anything on your site changing.
Why it matters for your rankings
Core updates are the largest single source of unexplained traffic movement. Google's own guidance is unusually direct: there is nothing specific to fix, because nothing is necessarily broken. Its assessment of what deserves to rank simply changed.
That advice frustrates people, but it does point at the right response. Recovery comes from making pages genuinely more useful and more clearly authored by people who know the subject, not from technical tweaks. It also takes time, often until the next core update, because the reassessment happens at that scale. The most damaging reaction is panic: rewriting titles, disavowing links and rolling back changes during a multi-week rollout adds noise and destroys your ability to tell what actually mattered.
How to check it on your site
Wait for the rollout to finish
Rankings fluctuate throughout the rollout window. Judge impact only after Google confirms completion.
Compare like periods
In Search Console compare the 28 days after full rollout against the 28 days before, by page and by query.
Read Google's quality questions
Google publishes a list of self-assessment questions for core updates. Apply them honestly to your worst-hit pages.
Check who replaced you
Search your lost queries and study the pages now ranking. The difference is usually depth, first-hand experience or clearer authorship.