textdomain

Textdomain is a label used in software internationalization to identify a specific collection of translated strings, stored in a translation catalog. The domain name corresponds to the filename portion of translation files (such as PO and MO files) and is used by translation functions to fetch the correct localized text for a given language. The domain helps organize translations by component, module, or project.

In PHP's gettext implementation, the translation process relies on functions like bindtextdomain and textdomain. Bindtextdomain(domain, directory) specifies where to find the catalog for a given domain, while textdomain(domain) sets the default domain for subsequent calls to translation functions such as gettext(), _, and ngettext(). Translation files are typically organized under locale/<language>/LC_MESSAGES/<domain>.mo, with the corresponding human-readable PO files used during development. If no domain is set, a fallback domain such as "messages" may be used by some environments, though behavior varies by platform.

Beyond PHP, the concept appears in other ecosystems that use gettext-style tooling, including content management systems and frameworks. In these contexts, a textdomain often corresponds to a particular component (for example, a plugin or theme) and is used when loading translations or when calling translation helpers like __() or _e() with an explicit domain. Correctly naming, loading, and referencing the domain ensures that translations are isolated, correctly localized, and not mixed across components.