Anidentifier is a name that identifies (that is, labels the identity of) either a unique object or a uniqueclass of objects, where the "object" or class may be an idea, person, physicalcountable object (or class thereof), or physicalnoncountable substance (or class thereof). The abbreviationID 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 encoding system (wherein letters, digits, words, or symbolsstand 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 acode orID code. For instance theISO/IEC 11179 metadata registry standard defines a code assystem 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 bearbitrary 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.)
Theunique identifier (UID) is an identifier that refers toonly one instance—only one particular object in the universe. Apart number is an identifier, but it is not aunique identifier—for that, aserial number is needed, to identifyeach instance of the part design. Thus theidentifier "Model T" identifies theclass(model) of automobiles that Ford'sModel T comprises; whereas theunique 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 ofname andidentifier aredenotatively equal, and the terms are thus denotativelysynonymous; but they are not alwaysconnotatively synonymous, becausecode names andID numbers are often connotatively distinguished from names in the sense of traditionalnatural 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 anemic indistinction rather than anetic one.
In metadata, an identifier is a language-independent label, sign or token that uniquely identifies an object within anidentification scheme. The suffix "identifier" is also used as arepresentation term when naming adata element.
ID codes may inherently carrymetadata 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. theGerman tank problem). Opaque identifiers—identifiers designed to avoid leaking even that small amount of information—include "reallyopaque pointers" andVersion 4 UUIDs.
Incomputer science, identifiers (IDs) arelexicaltokens that nameentities. 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.
Incomputer languages, identifiers aretokens (also calledsymbols) which name language entities. Some of the kinds of entities an identifier might denote includevariables,types,labels,subroutines, andpackages.
A resource may carry multiple identifiers. Typical examples are:
The inverse is also possible, where multiple resources are represented with the same identifier (discussed below).
Manycodes andnomenclatural systems originate within a smallnamespace. 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 necessitatingretronymy,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 (context was formerly assumed, and narrow), lack of capacity (e.g., low number of possible IDs, reflecting the outmoded narrow context), lack ofextensibility (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 callednaming collision. The story of the origination and expansion of theCODEN system provides a good case example in a recent-decades, technical-nomenclature context. The capitalization variations seen withspecific designators reveals an instance of this problem occurring innatural languages, where the proper noun/common 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.
| Identifier | Scope |
|---|---|
| atomic number, corresponding one-to-one withelement name | international (viaISV) |
| Australian Business Number | Australian |
| CAGE code | U.S. andNATO |
| CAS registry number | originated in U.S.; today international (viaISV) |
| CODEN | originated in U.S.; today international |
| Digital object identifier (DOI, doi) | Handle SystemNamespace, international scope |
| DIN standard number | originated in Germany; today international |
| E number | originated in E.U.; may be seen internationally |
| 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 theEANNamespace; international scope |
| International eBook Identifier Number (IEIN) | international |
| International Standard Serial Number (ISSN) | international |
| 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 |
The dictionary definition ofidentifier at Wiktionary
Media related toIdentifiers at Wikimedia Commons