Definition
Means 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
Source
Point (12) of Article 4 of Regulation (EU) 2016/679 of the European Parliament and of the Council of 27 April 2016
Link
https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32016R0679