Pseudonymisation 

IMI Code of Practice: Process of removing all elements allowing the identification of an individual person, except the key(s) allowing linking the data to the person. Such key shall be randomly generated and subject to technical and organisational measures to prevent its unauthorised use. ISO/TS 25237:2008: particular type of anonymization that both removes the association with a data subject and adds an association between a particular set of characteristics relating to the data subject and one or more pseudonyms.