The Regulation defines “personal data” as any information relating to an identified or identifiable natural person (‘data subject’); an identifiable natural person is one who can be identified, directly or indirectly, in particular by reference to an identifier such as a name, an identification number, location data, an online identifier or to one or more factors specific to the physical, physiological, genetic, mental, economic, cultural or social identity of that natural person.
The University of Warwick has some useful online information around GDPR, including FAQs and a list of what could be considered personal data.