Pseudonymisation refers to the processing of data in such a way that it can no longer be assigned to a specific person without the involvement of further data. All other data must be stored and protected separately. Pseudonymisation is one of the possible measures of privacy by design. One example is the use of a nickname in social networks or forums.
An idea related to pseudonymisation is anonymisation. This means adapting personal data so that it can no longer be assigned to an individual person. True anonymisation can hardly be achieved without aggregation or falsification of data, so that (supposed) anonymisation can almost always be reversed if data sets are sufficiently large.
Article 4(5) GDPR (Definitions)
“pseudonymisation” means 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;
Source: Regulation (EU) 2016/679