The “jQuery is not defined” error usually means a script tried to use jQuery before jQuery loaded. In WordPress, that can happen after theme changes, plugin conflicts, script optimization, CDN issues, or custom code that bypasses the proper enqueue system.
Use WordPress enqueueing
WordPress has a dependency system for scripts. Themes and plugins should enqueue scripts and declare jQuery as a dependency when they need it. Hard-coding script tags in templates is a common way to create ordering problems.
Check optimization plugins
Minify, combine, defer, and delay settings can accidentally move jQuery after a script that depends on it. If the error appears after performance changes, disable script optimization on staging and re-enable settings one at a time.
Look for CDN or blocked files
If jQuery is loaded from an external CDN and that request fails, dependent scripts may break. Browser developer tools can show whether the file is missing, blocked, redirected, or returning the wrong content type.
Clear cache after fixes
After changing script settings, clear plugin cache, server cache, CDN cache, and browser cache. Old cached HTML can keep referencing an old script order.
Bottom line
Fix the root ordering problem. Enqueue scripts correctly, declare dependencies, be cautious with optimization settings, and verify in the browser console after cache clears.