A number, usually associated with a login name, used by the system to identify owners of files and directories. The UID of a process becomes the owner of files created by the process and descendant processes. 
Unique Identifier, details ...
User Identifier
Unidentified
General computing term used to refer to a number, such as a product code, registration code, or a text entry. In databases, the UID can also be referred to as the key field, a unique identifier to a particular record. If the user does not specify a key field, the software must generate a UID so that each entry is individually tagged