FORMAL ANSWER

A personal data breach is a breach of security leading to the accidental or unlawful destruction, loss, alteration, unauthorised disclosure of, or access to, personal data transmitted, stored or otherwise processed.

PRACTICAL ANSWER

A personal data breach is any incident that causes non-compliance with the principles of personal data processing, e.g. access by unauthorised persons, destruction of a laptop, modification of a bank account number for payments, leakage of email logins and passwords, or unlawful disclosure of personal data (e.g. photos) on a website.

MORE:

• Check which breaches must be reported to the Polish DPA: Personal data breach severity calculator