Pseudonymization is a data processing technique that replaces directly identifying information — names, email addresses — with an artificial identifier (a pseudonym), so data can no longer be attributed to a specific individual without access to separately stored mapping information. GDPR Article 4(5) defines it as a recognized privacy-enhancing measure; Article 25 recommends it as a Privacy by Design implementation. Pseudonymized data remains personal data under GDPR if re-identification is possible — the key distinction from anonymization, which removes GDPR obligations entirely. It can reduce breach notification risk and helps controllers satisfy data minimization requirements under GDPR Article 5(1)(c).