What is « page title not defined » in the Page titles report?
Matomo (Piwik) provides two reports for your page views: Page Titles and Page URLs. A page title is the content of the HTML <title> tag. If your pages do not define this <title> Matomo will instead report « page title not defined » for the visits on those pages.
We also highly recommend you use the latest version of the Javascript Tracking Tag in case you are still using the old Tracking tag.