The distinction stems partly from the US-EU divide. US use the term personally identifiable information (PII). This has a narrow definition and relates to that which connects your real identify. However, 'PII' is not a concept under EU data protection law which instead uses the far broader term of personal data.

Personal data is both PII and any information which could be used to identify someone when combined with other information. As you suggest, a username is likely to be personal data because in lots of cases it is used as a unique ID and is used by the same person across multiple sites to identify them.