Identifier

An identifier is a name used to identify a distinct entity in a system, such as a variable, function, type, label, or object. Identifiers provide a human-usable handle for a machine-usable reference, enabling binding, lookup, and disambiguation across software, databases, and mathematical models. The precise meaning and rules for identifiers depend on the domain and language in use.

In programming languages, identifiers denote program entities like variables, functions, classes, or modules. They must follow lexical rules that specify allowed characters, often letters, digits, and underscores, and may not begin with a digit. Most languages are case-sensitive and reserve certain words, called keywords, that cannot be used as identifiers. Unicode support is common in modern languages, allowing a wide range of scripts.

Identifiers have scope and binding. They exist within defined regions of code (local, block, module, or global) and are associated with bindings stored in symbol tables during compilation or interpretation. Identical names in different scopes refer to different entities, resolved according to the language’s scope rules.

In databases, identifiers name schema elements such as tables, columns, indexes, and constraints. SQL systems typically require quoting for reserved words or special characters and impose limits on length and character sets. Beyond computing, identifiers appear in mathematics and formal languages as symbols representing variables or constants; in metadata and information systems, identifiers such as DOIs or URIs provide persistent, global references.

Best practices include choosing meaningful, consistent names, avoiding reserved words, and preventing collisions through clear naming conventions and namespaces. An identifier is therefore a concise, unambiguous label that uniquely refers to an element within its system.