Personally identifiable information
Also known as PII
Any data that can identify a specific person, either alone or combined with other available information.
Direct identifiers such as name and social security number are obvious. The harder category is data that identifies someone in combination, where a date of birth, postcode, and job title together single out one individual.
Different regimes define it differently, which is why data classification should be based on your actual obligations rather than a generic list.
Where this comes up
Personally identifiable information sits inside our compliance and frameworks work. If you are trying to work out what this means for your own environment rather than in the abstract, that is the page worth reading next, and a short call will get you a straight answer faster than either.