[Keefe]

Short for security identifier, a security feature of the Windows NT and 2000 operating systems. The SID is a unique name (alphanumeric character string) that is used to identify an object, such as a user or a group of users in a network of NT/2000 systems.
Windows grants or denies access and privileges to resources based on ACLs, which use SIDs to uniquely identify users and their group memberships. When a user requests access to a resource, the user’s SID is checked by the ACL to determine if that user is allowed to perform that action or if that user is part of a group that is allowed to perform that action.