WordPress allows you to set a ‘category base’ to show in the permalinks of the category archive pages – you can set this under ‘dashboard – settings – permalinks – category base’.
However, the latest default theme Twenty Fifteen is using a function to generate the Category Archive page title – get_the_archive_title() – which simply output ‘Category’ and does not reflect on the ‘category base’ entry.
Luckily, the function applies a filter which we can use to influence the Category Archive page title.
Example of a filter function:
add_filter( 'get_the_archive_title', 'use_category_base_in_archive_title' );
function use_category_base_in_archive_title( $title ) {
if ( is_category() && get_option('category_base') ) $title = str_replace( 'Category:', ucfirst( get_option('category_base') ) . ':', $title );
return $title;
}
As the category base is usually all lowercase letters, the suggested code also capitalizes the first letter when used as Category Archive page title.