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.

Unique identifiers can be assigned to anything that needs to be distinguished from other entities, such as individual users, companies, machines or websites. These distinctive values are usually assigned depending on the needs of the specific application, but can either be randomly auto-generated with an algorithm, allocated incrementally or chosen by the user.

Uses of UIDs

The most widely known use of unique identifiers occurs when users register for a website or service. Customers are often provided with a username or user ID that allows the company they are registering with to differentiate them within their user logs. These identifiers are then also used for security and log on purposes.

In a database or spreadsheet, unique identifiers may be designated as a specific column or field to help make sorting and filtering through information easier. This also helps trace information back to a specific user or entity within the system.

Another popular application of UIDs is in a physical supply chain. Manufacturers often mark individual pieces of a larger component, such as computer parts, or an entire product with a serial number. This allows users to trace back the origin of the product in case of a malfunction, defect or recall.

Unique identifiers are also commonly used within the healthcare industry. Instead of reporting medical information with a patient’s name, a personal code is created. This code combats issues of patient privacy as well as eliminates the possibility of duplicate reports.

Examples of UIDs

While UIDs can take a variety of forms depending on the application, a few examples include:

  • 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.
  • A 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.
  • A MAC address is a computer’s unique hardware number in a local area network (LAN).

Other, more basic ways to uniquely identify an entity or user include by social security number, email address, username and phone number.


Within the Department of Defense (DoD), there are three related acronyms associated with the standard of government property. The first is the Item Unique Identification (IUID) Registry. This registry stores information for the DoD about how to track, maintain and deploy government assets. Items are added to the IUID Registry if their acquisition cost is over five thousand dollars, they are mission critical or they are high risk.

The second and third acronyms are connected with each item in the registry. An item’s UID is the physical marking on the item, typically in the form of a barcode, that distinguishes it from other items. An item’s unique item identifier (UII) refers to its entry value in a database. UIIs are usually machine-readable and help contractors track each item digitally through all stages of its lifecycle.

This was last updated in September 2019

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