Google Trends is a website hosted by Google that allows you to get a measurement of Google search volume for a term. You can try it yourself here: https://trends.google.com/trends/.
Example of Google Trends volume for “european union” from the United Kindgom around the Brexit vote.
Nowcasting is predicting the present. It provides an estimation of the value of a quantity based on signals that appear at the same time. Nowcasting is valuable when data reporting is delayed, for example in the case of flu incidence.
Google Flu Trends aimed at nowcasting influenza-related physician visits based on Google search volumes. The reports from the Centers for Disease Control are delayed by two weeks, making this a good case for nowcasting. Google Flu Trends used a set of 45 search terms as predictors from a set of 50 million candidate terms.
As reported in the Nature paper, Google Flu Trends achieved a high weekly accuracy between 2003 and 2008. It was applied worked for various regions in the US, not just the total. The figure above shows the nowcast of Google Flu Trends two weeks ahead of the report from the CDC.
In February 2013, Google Flu Trends started overestimating the incidence of flu cases.