Security Identifier

In the context of the Microsoft Windows NT line of operating systems, a Security Identifier (commonly abbreviated SID) is a unique, immutable identifier of a user, user group, or other security principal. A security principal has a single SID for life, and all properties of the principal, including its name, are associated with the SID. This design allows a principal to be renamed (for example, from "John" to "Jane") without affecting the security attributes of objects that refer to the principal.