  Moving toward the bottom of the diagram, there is a region marked with a green dotted line labelled “truly feasible”.  Note that this is not a mathematically defined class, but rather an intuitive notion of those problems that can be solved exactly, for all the instances of reasonable size, within a reasonable amount of time, using a computer that we can afford. (Interestingly, as the speed of computers has dramatically increased over the years, our expectation of how large an instance we should be able to handle has increased accordingly.  Thus, the boundary of what is “truly feasible” changes more slowly than the increase of computer speed might suggest.)