What does it mean for something to be not a thing?

This question comes to me when I see people talking about things like tree spirits. So what is the definition of a thing? tree spirits are not tangible, and not visible. So what is the concept of an immaterial thing? Is it something that doesn’t exist?<br><br>Example: Forest spirits are spiritual beings believed to inhabit forests in one form or another. -wikipedia<br><br>They are often thought of as tree smites. <br><br><br>I’m not talking about Forest spirits, I am talking about the concept of immaterial thing. What does that mean? <br><br>Does something need to exist to be a thing?