WordPress
Taxonomy
Taxonomy is a named system WordPress uses to group content, such as categories and tags. You can register your own and attach them to any post type.
How it is measured
Core ships category, post_tag, post_format, link_category, and nav_menu. You add one with register_taxonomy( 'venue', 'event', $args ). The data lives in three tables: wp_terms for names, wp_term_taxonomy for the taxonomy and counts, and wp_term_relationships linking posts to terms.
List them with wp taxonomy list and the terms with wp term list venue. The hierarchical argument decides whether terms nest like categories or sit flat like tags.
Worked example
A record label's site has an album post type and wants to filter by genre and by decade. The developer registers a hierarchical genre taxonomy with 24 terms and a flat decade taxonomy with 6. A rock subgenre is nested under Rock.
The archive at /genre/shoegaze/ shows 38 albums. After a bulk import, the term count in wp_term_taxonomy says 31 until the developer runs wp term recount genre.
How it differs
A taxonomy groups items, while a custom post type defines what the items are. Events are a type and Venue is a taxonomy that classifies them. A taxonomy can attach to several types, and a type can use several taxonomies. Neither one stores the other's content.
Common errors
Using categories for everything and ending with 300 near-duplicate terms. Using a tag as a category. Registering a taxonomy without show_in_rest so it does not appear in the block editor. Changing a taxonomy's slug and breaking the archive URLs. Deleting a term and leaving posts without a default.
In practice
Write down the groupings you actually filter by. Make each a taxonomy with a clear slug and a fixed list of terms. Merge duplicates with a bulk edit in wp-admin, and run wp term recount after large imports.