A unique identifier or set of data used to recognize or verify an entity within a particular system. An ID can be a combination of numbers or letters and is often used to identify a person, device, object, or transaction. In the context of information technology, an ID can be used to identify user accounts, database entries, web page elements, and more. A user ID is often used in combination with a password to control access to specific systems or services. It is important to manage IDs securely to ensure privacy and security.