What Is Re-identification?

Re-identification — the process or risk of linking supposedly anonymous data back to the individuals it describes.

Datasets stripped of obvious identifiers can often still be re-identified by combining them with other available data — a recurring finding in privacy research. Re-identification risk is why "we removed the names" is not the same as anonymisation, and why releasing or training on de-identified data still carries privacy obligations.

Source: Privacy research; ICO guidance