Definition
Means a supervisory authority which is concerned by the processing of personal
data because:
(a) the controller or processor is established on the territory of the Member State of that supervisory authority;
(b) data subjects residing in the Member State of that supervisory authority are substantially affected or likely to be substantially affected by the processing; or
(c) a complaint has been lodged with that supervisory authority
Source
Point (22) 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
