Refers to a box-like reusable user-interface component common in the WordPress backend.

A Metabox is a box-like "frame" used extensively in the WordPress backend by both the WordPress Core and by plugins and themes. A meta box can contain many different kinds of content but are probably most associated with the post edit screens and boxes identifiable by headers such as "Publish" and "Custom Fields". Metaboxes provide a convenient and consist mechanism for plugin and theme authors to incorporate additional data into the WordPress post edit screen and other administrative pages.

The primary functions for manipulating metaboxes are add_meta_box and remove_meta_box.