The process of associating a particular piece of data with a particular entity (person, household, business etc). Note that attribution can happen without reidentification (if for example all members of a group share a common attribute).
The GDPR only discusses attribution in the context of pseudonymisation. While personal data relate directly to an individual, pseudonymised data can only be ‘attributed’ to an individual with the use of auxiliary information (which must be held separately).
See also: ATTRIBUTE DISCLOSURE, IDENTIFIABLE DATA