An identifier is a name that identifies either a unique object or a unique class of objects, where the "object" or class may be an idea, physical object, or physical substance. The abbreviation ID often refers to identity, identification, or an identifier. An identifier may be a word, number, letter, symbol, or any combination of those.
The words, numbers, letters, or symbols may follow an encoding system or they may simply be arbitrary. When an identifier follows an encoding system, it is often referred to as a code or ID code. For instance the ISO/IEC 11179 metadata registry standard defines a code as system of valid symbols that substitute for longer values in contrast to identifiers without symbolic meaning. Identifiers that do not follow any encoding scheme are often said to be arbitrary IDs; they are arbitrarily assigned and have no greater meaning.
The unique identifier is an identifier that refers to only one instance—only one particular object in the universe. A part number is an identifier, but it is not a unique identifier—for that, a serial number is needed, to identify each instance of the part design. Thus the identifier "Model T" identifies the class ' of automobiles that Ford's Model T comprises; whereas the unique identifier "Model T Serial Number 159,862" identifies one specific member of that class—that is, one particular Model T car, owned by one specific person.
The concepts of name and identifier are denotatively equal, and the terms are thus denotatively synonymous; but they are not always connotatively synonymous, because
code names and ID numbers''' are often connotatively distinguished from names in the sense of traditional natural language naming. For example, both "Jamie Zawinski" and "Netscape employee number 20" are identifiers for the same specific human being; but normal English-language connotation may consider "Jamie Zawinski" a "name" and not an "identifier", whereas it considers "Netscape employee number 20" an "identifier" but not a "name". This is an emic indistinction rather than an etic one.


In metadata, an identifier is a language-independent label, sign or token that uniquely identifies an object within an identification scheme. The suffix "identifier" is also used as a representation term when naming a data element.
ID codes may inherently carry metadata along with them. For example, when you know that the food package in front of you has the identifier "2011-09-25T15:42Z-MFR5-P02-243-45", you not only have that data, you also have the metadata that tells you that it was packaged on September 25, 2011, at 3:42pm UTC, manufactured by Licensed Vendor Number 5, at the Peoria, IL, USA plant, in Building 2, and was the 243rd package off the line in that shift, and was inspected by Inspector Number 45.
Arbitrary identifiers might lack metadata. For example, if a food package just says 100054678214, its ID may not tell anything except identity—no date, manufacturer name, production sequence rank, or inspector number. In some cases, arbitrary identifiers such as sequential serial numbers leak information. Opaque identifiers—identifiers designed to avoid leaking even that small amount of information—include "really opaque pointers" and Version 4 UUIDs.

In computer science

In computer science, identifiers are lexical tokens that name entities. Identifiers are used extensively in virtually all information processing systems. Identifying entities makes it possible to refer to them, which is essential for any kind of symbolic processing.

In computer languages

In computer languages, identifiers are tokens which name language entities. Some of the kinds of entities an identifier might denote include variables, types, labels, subroutines, and packages.


Identifiers (IDs) versus Unique identifiers (UIDs)

Many resources may carry multiple identifiers. Typical examples are:
The inverse is also possible, where multiple resources are represented with the same identifier.

Implicit context and namespace conflicts

Many codes and nomenclatural systems originate within a small namespace. Over the years, some of them bleed into larger namespaces. When such dissemination happens, the limitations of the original naming convention, which had formerly been latent and moot, become painfully apparent, often necessitating retronymy, synonymity,
translation/transcoding, and so on. Such limitations generally accompany the shift away from the original context to the broader one. Typically the system shows implicit context, lack of capacity, lack of extensibility, and lack of specificity and disambiguating capability. Within computer science, this problem is called naming collision. The story of the origination and expansion of the CODEN system provides a good case example in a recent-decades, technical-nomenclature context. The capitalization variations seen with specific designators reveals an instance of this problem occurring in natural languages, where the proper noun/common noun distinction must be dealt with. A universe in which every object had a UID would not need any namespaces, which is to say that it would constitute one gigantic namespace; but human minds could never keep track of, or semantically interrelate, so many UIDs.

Identifiers in various disciplines

atomic number, corresponding one-to-one with element nameinternational
Australian Business NumberAustralian
CAGE codeU.S. and NATO
CAS registry numberoriginated in U.S.; today international
CODENoriginated in U.S.; today international
Digital object identifier Handle System Namespace, international scope
DIN standard numberoriginated in Germany; today international
E numberoriginated in E.U.; may be seen internationally
EC number
Employer Identification Number U.S.
Electronic Identifier Serial Publicaction international
Global Trade Item Numberinternational
Group identifiermany scopes, e.g., specific computer systems
International Chemical Identifierinternational
International Standard Book Number ISBN is part of the EAN Namespace; international scope
International eBook Identifier Number international
International Standard Serial Number international
ISO standard number, e.g., ISO 8601international
Library of Congress Control NumberU.S., with some international bibliographic usefulness
Personal identification numbermany scopes, e.g., banks, governments
Personal identification number Denmark
Pharmaceutical codeMany different systems
Product batch number
Serial Item and Contribution IdentifierU.S., with some international bibliographic usefulness
Serial numbermany scopes, e.g., company-specific, government-specific
Service batch number
Social Security NumberU.S.
Tax file numberAustralian
Unique Article Identifier international