GDPR Definition
"Any information relating to an identified or identifiable natural person" - including names, ID numbers, location data, online identifiers, and factors specific to physical, physiological, genetic, mental, economic, cultural or social identity.
Special Consideration for Small Populations
In small organizations or communities, demographic characteristics that wouldn't normally identify someone can become PII. Being the only person of a certain ethnicity, gender, or age group in a small population can make someone easily identifiable even without their name.