Site migration
Any large change to a site's URLs, domain, platform or structure that search engines must re-learn.
Domain moves, HTTPS switches, platform changes and restructures all count. The essentials are a complete map of old URLs to new ones with permanent redirects, updated internal links and sitemaps, and — for a domain move — Search Console's change of address tool.
Temporary ranking fluctuation after a migration is normal; a lasting drop usually traces back to missing redirects or lost content.
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