Which term describes the number used by a computer to recognize a user account?

The term "User ID" specifically refers to the unique identifier assigned to a user account within a computer system. It is crucial for the identification and authentication processes, allowing the system to differentiate between different users and manage their access rights and privileges effectively. The User ID is often combined with a password to validate the identity of the user when logging into a system.

While "Account key," "Identifier," and "User reference" could imply an identification mechanism or elements related to accounts, they do not specifically capture the standard terminology widely used in computing contexts to denote a user's unique identification string. Therefore, "User ID" precisely fits the definition and common usage in information security and computing.