A persistent identification element (PIE) is an online identifier that is intended to be used for a long period of time, and across multiple platforms and services.
PIEs are typically assigned by an authority, such as a website or an online service provider, and they are meant to be stable and resilient. This stability is important in order to allow users to build up a history and reputation online, without having to worry about their PIE being changed or revoked.
There are a few different types of PIEs, but the most common are usernames, email addresses, and social media handles. All of these can be used to identify a user across multiple platforms and services, and they are all intended to be persistent over time.