JavaScript's ReferenceError: $ is not defined occurs when the jQuery library, often represented by the $ symbol, is not properly loaded or available in the code's current scope. This error typically arises from the omission of the jQuery library in a script, typographical errors, or an incorrect script execution order where the script runs before the library is loaded. To correct this, jQuery must be included before any dependent code, either via downloading and hosting it locally or using a content delivery network (CDN) link in the HTML file. An example highlights this error occurring when jQuery is not loaded, and a corrected script shows its inclusion via a CDN link to prevent the error. Additionally, managing JavaScript errors can be streamlined with tools like Rollbar, which provides automated error monitoring and triaging to simplify the debugging process.