Editing Identifier

{{Short description|Name that identifies a specific entity}}
[[File:4373 Russian license plate Toyota car.jpg|thumb|400x400px|alt=A Russian license plate with the value "C051KO199" is mounted on the front grille of a Toyota car|[[Vehicle registration plate|Registration plates]] are used to display identifiers for [[Motor vehicle|motor vehicles]].]]

An '''identifier''' is a name that identifies (that is, labels the identity of) either a unique object or a unique ''class'' of objects, where the "object" or class may be an idea, person, physical [[countable]] object (or class thereof), or physical [[mass noun|noncountable]] substance (or class thereof). The abbreviation '''ID''' often refers to identity, identification (the process of identifying), or an identifier (that is, an instance of identification). An identifier may be a word, number, letter, symbol, or any combination of those.

The words, numbers, letters, or symbols may follow an [[code|encoding system]] (wherein letters, digits, words, or symbols ''stand for'' [represent] ideas or longer names) or they may simply be arbitrary. When an identifier follows an encoding system, it is often referred to as a '''code''' or '''id&nbsp;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&nbsp;Ids'''; they are arbitrarily assigned and have no greater meaning. (Sometimes identifiers are called "codes" even when they are actually arbitrary, whether because the speaker believes that they have deeper meaning or simply because they are speaking casually and imprecisely.)

The [[unique identifier]] ('''UID''') 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&nbsp;T" identifies the ''class'' ''(model)'' of automobiles that Ford's [[Ford Model T|Model&nbsp;T]] comprises; whereas the ''unique identifier'' "Model&nbsp;T&nbsp;Serial Number&nbsp;159,862" identifies one specific member of that class—that is, one particular Model&nbsp;T car, owned by one specific person.

The concepts of ''name'' and ''identifier'' are [[denotation|denotatively]] equal, and the terms are thus denotatively [[synonym]]ous; but they are not always [[connotation|connotatively]] synonymous, because '''[[code name]]s''' 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&nbsp;employee&nbsp;number&nbsp;20" an "identifier" but not a "name." This is an [[emic]] indistinction rather than an [[etic]] one.

==Metadata==
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 (i.e. the [[German tank problem]]). Opaque identifiers—identifiers designed to avoid leaking even that small amount of information—include "really [[opaque pointer]]s" and [[universally unique identifier#Version 4 (random)|Version 4 UUIDs]].

==In computer science==
<!-- [[Identifier (computer science)]] redirects here -->

In [[computer science]], identifiers (IDs) are [[Lexical (semiotics)|lexical]] [[Token (parser)|tokens]] that name [[Entity class|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===
{{Main|Identifier (computer languages)}}
In [[computer language]]s, identifiers are [[Token (parser)|tokens]] (also called [[symbol]]s) which name language entities. Some of the kinds of entities an identifier might denote include [[variable (programming)|variables]], [[data type|types]], [[label (programming language)|labels]], [[subroutine]]s,  and [[modular programming|packages]].

==Ambiguity==
===Identifiers (IDs) versus Unique identifiers (UIDs)===
{{Main|Unique identifier}}
A resource may carry multiple identifiers. Typical examples are:
* One person with multiple names, nicknames, and forms of address (titles, salutations)
** ''For example:'' One specific person may be identified by all of the following identifiers: Jane&nbsp;Smith; Jane&nbsp;Elizabeth&nbsp;Meredith&nbsp;Smith; Jane&nbsp;E.&nbsp;M.&nbsp;Smith; Jane&nbsp;E.&nbsp;Smith; Janie&nbsp;Smith; Janie; Little&nbsp;Janie (as opposed to her mother or sister or cousin, Big&nbsp;Janie); Aunt&nbsp;Jane; Auntie&nbsp;Janie; Mom; Grandmom; Nana; Kelly's&nbsp;mother; Billy's&nbsp;grandmother; Ms.&nbsp;Smith; Dr.&nbsp;Smith; Jane&nbsp;E.&nbsp;Smith,&nbsp;PhD; and Fuzzy (her jocular nickname at work).
* One document with multiple versions<ref name=paid>{{cite web
  | last = University of Glasgow
  | author-link = University of Glasgow
  | title = Procedure for Applying Identifiers to Documents
  | url = http://www.gla.ac.uk/infostrat/ERM/Docs/procref.htm
  | access-date = 28 April 2009
  | archive-url = https://web.archive.org/web/20110605081415/http://www.gla.ac.uk/infostrat/ERM/Docs/procref.htm
  | archive-date = 5 June 2011
  | url-status = dead
  }}</ref>
* One substance with multiple names (for example, [[Chemical Abstracts Service|CAS]] index names versus [[IUPAC]] names;<ref name=icn>{{cite web
  | last = University of Pennsylvania
  | author-link = University of Pennsylvania
  | title = Information on Chemical Nomenclature
  | url = http://gethelp.library.upenn.edu/guides/scitech/chemnom.html
  | access-date = 28 April 2009
  | archive-date = 4 January 2009
  | archive-url = https://web.archive.org/web/20090104023819/http://gethelp.library.upenn.edu/guides/scitech/chemnom.html
  | url-status = dead
  }}</ref> [[International Nonproprietary Name|INN]] generic drug names versus [[United States Adopted Name|USAN]] generic drug names versus brand names)

The inverse is also possible, where multiple resources are represented with the same identifier (discussed below).

===Implicit context and namespace conflicts===
{{see also|Naming collision}}
Many [[code]]s and [[nomenclature|nomenclatural]] systems originate within a small [[namespace]]. Over the years, some of them bleed into larger namespaces (as people interact in ways they formerly had not, e.g., cross-border trade, scientific collaboration, military alliance, and general cultural interconnection or assimilation). When such dissemination happens, the limitations of the original naming convention, which had formerly been latent and moot, become painfully apparent, often necessitating [[retronym]]y, [[synonym]]ity,
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 (context was formerly assumed, and narrow), lack of capacity (e.g., low number  of possible IDs, reflecting the outmoded narrow context), lack of [[extensibility]] (no features defined and reserved against future needs), and lack of specificity and disambiguating capability (related to the context shift, where longstanding uniqueness encounters novel nonuniqueness). 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 [[proper noun#Specific designators|specific designators]] reveals an instance of this problem occurring in [[natural language]]s, where the proper&nbsp;noun/common&nbsp;noun distinction (and its complications) 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==
{{Main|:Category:Identifiers}}
{| class="wikitable"
|-
! Identifier !! Scope
|-
| [[atomic number]], corresponding one-to-one with [[List of chemical element name etymologies|element name]] || international (via [[International scientific vocabulary|ISV]])
|-
| [[Australian Business Number]] || Australian
|-
| [[Commercial and Government Entity|CAGE code]] || U.S. and [[NATO]]
|-
| [[CAS registry number]] || originated in U.S.; today international (via [[International scientific vocabulary|ISV]])
|-
| [[CODEN]] || originated in U.S.; today international
|-
| [[Digital object identifier]] (DOI, doi) || [[Handle System]] [[Namespace]], international scope
|-
| [[Deutsches Institut für Normung|DIN]] standard number || originated in Germany; today international
|-
| [[E number]] || originated in E.U.; may be seen internationally
|-
| [[Enzyme Commission number|EC number]] ||
|-
| [[Employer Identification Number]] (EIN) || U.S.
|-
| Electronic Identifier Serial Publicaction (EISP) || international
|-
| [[Global Trade Item Number]] || international
|-
| [[Group identifier]] || many scopes, e.g., specific computer systems
|-
| [[International Chemical Identifier]] || international
|-
| [[International Standard Book Number]] (ISBN) || ISBN is part of the [[International Article Number (EAN)|EAN]] [[Namespace]]; international scope
|-
| International eBook Identifier Number (IEIN) || international
|-
| [[International Standard Serial Number]] (ISSN) || international
|-
| [[International Organization for Standardization|ISO]] standard number, e.g., [[ISO 8601]] || international
|-
| [[Library of Congress Control Number]] || U.S., with some international bibliographic usefulness
|-
| [[Personal identification number (Denmark)]] || Denmark
|-
| [[Pharmaceutical code]] || Many different systems
|-
| Product batch number ||
|-
| [[Serial Item and Contribution Identifier]] || U.S., with some international bibliographic usefulness
|-
| [[Serial number]] ||  many scopes, e.g., company-specific, government-specific
|-
| Service batch number ||
|-
| [[Social Security Number]] || U.S.
|-
| [[Tax file number]] || Australian 
|-
| Unique Article Identifier (UAI) || international
|-
|}

==See also==
{{columns-list|colwidth=30em|
* [[Barcode]]
* [[Binomial nomenclature]]
* [[British Approved Name]]
* [[Data descriptor]]
* [[Data element]]
* [[Descriptor (disambiguation)|Descriptor]]
* [[Diagnosis code]]
* [[Document management system]]
* [[File descriptor]]
* [[Food labeling regulations]]
* [[Gene nomenclature]]
* [[Handle (computing)]]
* [[Identification (disambiguation)|Identification]]
* [[Identity (object-oriented programming)]]
* [[Identity document]]
* [[Index term]]
* [[Marketing part number]]
* [[Metadata]]
* [[Name binding]]
* [[Namespace]]
* [[Naming convention (programming)]]
* [[National identification number]]
* [[Nomenclature]] – contains various standardized naming systems
* [[Nomenclature code]]
** [[Chemical nomenclature]]
** [[International Code of Nomenclature for algae, fungi, and plants]]
** [[International Code of Nomenclature of Bacteria]]
** [[International Code of Nomenclature for Cultivated Plants]]
** [[International Code of Zoological Nomenclature]]
* [[Function overloading|Overloading]]
* [[Part number]]
* [[Personally identifiable information]]
* [[Product code (disambiguation)|Product code]]
* [[Reference (computer science)]]
* [[Referent]]
* [[Representation term]]
* [[Systematized Nomenclature of Medicine]]
* [[Uniform resource identifier]] (URI)
* [[Unique identifier]]
* [[Unique key]]
}}

==References==
{{Reflist}}

==External links==
*{{Wiktionary-inline}}
*{{Commons category-inline|Identifiers}}

{{Personal names}}
{{Authority control}}

[[Category:Identifiers| ]]
[[Category:Metadata]]