Google Dance was the term given to the period of time when Google published updates to it's index of the world wide web. When this update occurred every 36 days (approximately 10 times per year) it was noticeable to those who looked hard enough because each Google server would display a different number of results for the same search query. However, Google now updates it's index once per week and so the change is considerably less noticeable and the term has fallen out of use. Check out this link https://www.webopedia.com/TERM/G/Google_Dance.html and this link https://metamend.com/archive/education/google-dance/ for more information