Irreversibly processing data so that individuals can no longer be identified, which takes it outside the scope of the GDPR.
Anonymization removes or alters data so that no one can be re-identified by any reasonably likely means. Because truly anonymized data is no longer personal data, the GDPR no longer applies to it.
Achieving genuine anonymization is difficult, since combining data sets can sometimes re-identify people. This is the key difference from pseudonymization, which is reversible.
Related frameworks
KaitoSec