PII is short for Personally Identifiable Information and generally refers to data that contains personally identifiable information, such as login info, emails, phone numbers, names, social handles, etc… They can be used to link together other identifiers and are a highly reliable, deterministic identifier.
Because it is information that has been entered by the user, it is often very reliable. It can also be used to bridge the online/offline identification challenge. PII need to conform to the growing demands of privacy legislation such as GDPR.
