What does the error message "ReferenceError: Schema is not defined" mean when using Mongoose and creating a new Schema?

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The error message "ReferenceError: Schema is not defined" means that the Schema object has not been properly imported or declared in the file where it is being used. This error message usually occurs when there is a problem with the Mongoose module or file structure. To fix this error, make sure that the Mongoose module is properly installed and imported into the file, and that the Schema object is declared and defined in the file before it is being used.