It is any accidental or unlawful destruction, loss, alteration, unauthorised disclosure of, or access to personal data transmitted, stored or otherwise processed in connection with the provision of a public electronic communications service. 

An example of a data breach is when an unauthorised person gets access to personal data or even access to the organisation. The definition also encompasses any accidental deletion or alteration of data by an employee of the Data Controller or Processor. 

This information is provided for guidance and cannot be classified as official legal advice. For official direction related to your obligations under GDPR, please speak to your legal counsel.