Builtin

Built-in refers to components, functionality, or data that are provided as an integral part of a system, language, or environment, rather than something created or added by the user. Built-ins are typically available without requiring separate installation or external modules.

In programming languages, built-ins include primitive types and a core set of functions, operators, and exceptions that are guaranteed to exist in every runtime. For example, Python exposes a builtins namespace with functions like len and print, which are available without importing modules. JavaScript provides a set of global objects and functions—such as Object, Array, Math, and Date—that are available in all execution contexts. Some languages organize built-ins as part of the standard library rather than as intrinsic features.

Command shells and runtime environments distinguish between built-in commands and external programs. In Bash, for instance, echo is a built-in command implemented by the shell, whereas many other utilities are separate executables found in the filesystem. The builtin command can be used to invoke a shell builtin explicitly, even if a function or alias with the same name exists.

Compiler and runtime implementations sometimes expose built-in functions or intrinsics that map directly to processor instructions or optimized routines. These built-ins enable low-level optimizations or platform-specific functionality while remaining part of the language’s standard interface.

The term also appears as a naming convention in libraries and tools to indicate functionality that is always present by design. Because what counts as a built-in can vary across languages and versions, reliance on specific built-ins can affect portability and long-term compatibility.