Pseudonymisation

Definition in the Law art. 4

the processing of personal data in such a manner that the personal data can no longer be attributed to a specific data subject without the use of additional information, provided that such additional information is kept separately and is subject to technical and organisational measures to ensure that the personal data are not attributed to an identified or identifiable natural person;

In plain words

Pseudonymisation is processing data so that it can no longer be attributed to a specific person without additional information, which is kept separately and protected by technical and organisational measures. A company that replaces names in its customer records with codes, and keeps the key separately and protected, pseudonymises the data.