Personal data is any information that on its own, or together with other information, can lead to the identification of an individual
Encrypted or pseudonymisation may still be classified as personal data when the encryption or pseudonymisation can be reversed, or the identification is still apparent despite processes used to hide them
Truly anonymous data is no longer considered personal data
References:
Article 2, Article 4(1) and (5) and Recitals (14), (15), (26), (27), (29) and (30) of the GDPR Article 29
Working Party Opinion 4/2007 on the concept of personal data
Article 29 Working Party Opinion 05/2014 on Anonymisation Techniques
Section 3 of the Data Protection Act2018 – interpretation and definitions