An industry nickname for Google’s April 2015 mobile-friendly ranking update for searches on mobile devices.
The dramatic nickname can make the change sound like a universal penalty. Google’s announcement described a mobile search update evaluated at the page level. A historical report should retain that scope rather than applying the name to every later mobile or desktop ranking change.
Mobile-friendly design and mobile-first indexing are also different ideas. One concerns usability; the other concerns which version of content is used for indexing. A responsive page can still have missing content or technical access problems that need separate investigation.
The lasting operational lesson is to make important tasks work on smaller screens. Read the page, open its navigation and submit a test form under an appropriate testing process. A desktop preview alone will not reveal every mobile interaction problem.
Takeaway: Treat mobile usability as part of the customer experience, not a one-time reaction to an old algorithm nickname.
Example
A service page fits the phone screen, but its quote form requires horizontal scrolling. The team repairs the form rather than assuming the responsive theme has completed all mobile work.
What to check
Check important pages individually and keep usability tests separate from indexing checks. Do not infer a current ranking penalty from the historical label.
For more detail, see Google mobile-friendly update announcement.