Pageview information about Wikimedia sites, which has been available for a while through the standalone Pageviews REST API [1], is now available in the action API as well, through the prop=pageviews [2], list=mostviewed [3] and meta=siteviews [4] API modules. An example:
https://en.wikipedia.org/w/api.php?action=query&format= jsonfm&errorformat=plaintext&prop=pageviews&list=mostviewed&meta=siteviews& titles=Main+Page&formatversion=2&pvipdays=3&pvimlimit=3&pvisdays=3
The new API modules basically act as a proxy to the REST API, and are meant as a less flexible but more convenient alternative for people who already use the action API.
[1] https://wikitech.wikimedia.org/wiki/Analytics/PageviewAPI [2] https://en.wikipedia.org/w/api.php?action=help&modules=query%2Bpageviews [3] https://en.wikipedia.org/w/api.php?action=help&modules= query%2Bmostviewed [4] https://en.wikipedia.org/w/api.php?action=help&modules=query%2Bsiteviews