Depending on the analytics provider that you use, you may see some of the following terms in your keyword data:

"Empty"

"Keyword Unavailable"

"Term Not Provided"

"None"​​​​​

This is a representation of Google's (not provided), which means that the entrance was not directly attributed to a single keyword as Google has limited the amount of those relationships it is showing.  

Why?  Google states, "As search becomes an increasingly customized experience, particularly for signed in users, we believe that protecting these personalized search results is important."  For more on this topic, please see Google's announcement (Making search more secure) and blog article (Making search more secure: Accessing search query data in Google Analytics).

We have a fantastic playbook around Term Not Provided and what we offer in our platform to help: Solution to Term Not Provided.