What is concept art

When I just started drawing I had a lot of difficulties understanding the difference between ‘normal’ art and concept art. To me, concept art more often than not looked finished and polished or fully rendered. While at the same time some specific art styles looked unfinished to me, or as something I’d expect to be a concept.
I know that with me there are far more people that don’t understand what concept art really is. And the truth is: You can’t tell by just looking at it. There is just one key ingredient to be able to tell if it’s concept art or not: Was it meant to serve as a concept or not.