Google Trends, Search Volume Index

The term Search Volume Index is a pretty tricky term and I am not exactly sure I have a full handle on it.  

Google Trends (Another Google Product) analyzes a portion of Google web searches to compute how many searches have been done for the terms you enter, relative to the total number of searches done on Google over time. We then show you a graph with the results – our Search Volume Index graph.

Here is a great article that discusses the Google Trends platform.