A data structure of variable length that uniquely identifies user, group, service, and computer accounts within an enterprise. Every account is issued a SID when the account is first created. Access control mechanisms in Windows 2000 identify security principals by SID rather than by name. See also relative ID; security principal.

A unique name that identifies a logged-on user to the security system of Windows NT. A security ID can identify either an individual user or a group of users.

A data structure of variable length that identifies user, group, and computer accounts. Every account on a network is issued a unique SID when the account is first created. Internal processes in Windows refer to an account's SID rather than the account's user or group name. See also: group account; group name; user account; user name