identifier

noun

1. Someone who identifies; a person who establishes the identity of.
2. Something that identifies or uniquely points to something or someone else.
3. A guidebook that helps determine the specific class of an object (such as a mushroom, herb, fish, bird, drug, or mineral), or its individual identity (such as that of a star).
4. (operating systems) A formal name used in source code to refer to a variable, function, procedure, package, etc. or in an operating system to refer to a process, user, group, etc.