unique identifier (UID)

A unique identifier (UID) is a numeric or alphanumeric string that is associated with a single entity within a given system. UIDs make it possible to address that entity, so that it can be accessed and interacted with. 

Here are a few examples of UIDs: 

• A Uniform Resource Identifier (URI) is a unique identifier that makes content addressable on the Internet by uniquely targeting items, such as text, video, images and applications.
• A Uniform Resource Locator (URL) is a particular type of URI that targets Web pages so that when a browser requests them, they can be found and served to users.
• A Universal Unique Identifier  (UUID) is a 128-bit number used to uniquely identify some object or entity on the Internet.
• A global unique identifier (GUID) is a number that Microsoft programming generates to create a unique identity for an entity such as a Word document.
• A bank identifier code  (BIC) is a unique identifier for a specific financial institution. 
• A unique device identifier (UDID) is a 40-character string assigned to certain Apple devices including the iPhone, iPad, and iPod Touch.
• An service set identifier (SSID) is a sequence of characters that uniquely names a wireless local area network (WLAN).
• A national provider identifier (NPI) is a unique ten-digit identification number required by HIPAA for all health care providers in the United States.