Simple definition
A unique identifier used by Windows to represent a user, group or other security principal.
Technical definition
Security Identifier is a structured value assigned to security objects. Windows permissions refer to the SID rather than the simple name displayed.
What is it used for ?
Maintaining a stable identity in the ACL even if the visible name of an account changes.
Practical example
Rename a user normally does not change its SID and his permissions remain associated.
Common issues
- Account deleted leaving a SID unknown in a ACL
- Duplication or cloning not properly managed
- Confusion between name and real identity
Related terms
Key takeaway: This entry explains the general operation of the term. Exact settings may vary depending on software, vendors and environments.