Refers to a mechanism built into the WP_Query class for querying the database for post data, including pages and CPTs, based upon post meta data.

WordPress stores post data in two tables primarily-- the *_posts table and the *_postmeta table-- not including the "categorization" data in the *_term* tables where category, tag, and custom taxonomy information is kept. Data in the *_postmeta table is considered "meta data". Some of this data is inserted and maintained by the WordPress Core, but meta data can be often is inserted by themes or plugins. With WordPress release 3.1 an enhanced mechanism for querying post meta data was introduced as part of the WP_Query class. To use this mechanism, WP_Query is passed an array with a key named meta_query. Such queries have come to be know as meta_queries or as a meta_query.