Personally identifiable information
Personally identifiable information (PII), also known as personally identifying information, is user data that can be used to contact the user, either directly or through a lookup.
User data that can be used to contact the user directly includes postal address and email address. User data that can be used, through a lookup, to contact the user includes SSN and other government issued ID numbers.
Some user data that does not fit either of these criteria is commonly considered PII because it has a reasonable probability of resulting in the ability to contact the user. The archetype for this is the user's full name. The intersection of large datasets with consistent cross-session identifiers are being increasingly shown to offer the opportunity to derive user contact details.