What is an identifier (Identifikator)?

An identifier is a designation, for instance a character string, that names a digital identity or a digital resource unambiguously within a namespace (domain); for resources it is often a URL or URI.

Uniqueness is always relative to the namespace, which is why the same username can exist at two organisations without conflict.

Go deeper:

• Identifier (Wikipedia) — names that label unique instances, and why uniqueness needs a scope.