Anonymisation / anonymous data
This is not a legal definition. It is described in Recital 26 of the GDPR; cf. non-personal electronic health data, Art. 2(2)(b) of the EHDS.
Processing data so that no one can reasonably still determine which person they concern. Recital 26 GDPR describes anonymous information as information that does not relate to an identified or identifiable natural person, or personal data rendered anonymous in such a way that the data subject is not or no longer identifiable; the GDPR does not apply to such data, including for statistical or research purposes. Whether someone is still identifiable depends on all objective factors, such as the cost of and time required for identification and available technology. Note the confusion: sometimes relatively anonymous is meant (anonymous to me) and sometimes absolutely anonymous (anonymous to everyone).
Questions about what this term means for your organisation? Contact Mr. Dr. Vlieger.