Definition
Personally identifiable information, or PII, is information that can identify, distinguish, trace, or be linked to a specific person, either on its own or when combined with other data. NIST SP 800-122 uses this formulation for US federal information-security practice.
Obvious examples include a name, government identifier, face image, or account number. Less obvious examples include precise location, device identifiers, rare attributes, free-text notes, and combinations of otherwise ordinary fields. Whether data is identifying depends on context, who can access it, and what other information they can join to it.
PII is not a universal legal category. Privacy laws use definitions such as personal data or personal information that may be broader, narrower, or structured differently. A glossary definition helps with system design, but it does not replace a jurisdiction-specific legal analysis.
Distinguish it from nearby terms
Direct identifiers point to a person on their own. Quasi-identifiers become identifying in combination. Sensitive data concerns the potential harm or protected nature of information; not all PII is equally sensitive, and some sensitive information may not identify a person.
Check your understanding
A dataset removes names but retains full birth dates, postal codes, job titles, and free-text notes. It may still contain PII because those fields can be linked back to individuals.