How do you know if a polygon is a polygon?

I'm curious, how does one actually determine if a given shape is indeed a polygon? Is it simply by counting the number of sides? Or are there specific criteria that must be met in order for a shape to be classified as a polygon? For instance, does it need to be closed, with straight lines connecting all the points? Are there any other defining characteristics that make a polygon, well, a polygon? I'm eager to hear your thoughts on this topic.