FAQ: What does "bundle" mean in WordPress terms?
In most contexts we simply mean "a theme or a plugin", but we've grown rather tired writing that all the time.
The WordPress core is also a bundle. It's a group of four sets of translations that all belong to the core WordPress codebase. In theory the term could apply to any application, module or add-on that runs inside WordPress and loads translations.
Although we don't like using a term outside the WordPress vernacular, we failed to find a widely accepted word that describes our concept abstractly and accurately enough. Bundle is also a term used in Symfony, so we thought it was a good fit for PHP developers.
What's wrong with "Text Domain"?
Themes and plugins often contain a single set of translations that live in their own "Text Domain", but some have many more. We've seen plugins that use up to 10 text domains. In theory there's no limit.
Furthermore, a text domain can be split into subsets. The WordPress core splits the
"default" text domain into three separate sets called "Development", "Admin" and "Network admin".