Rendering is basically adding detail and tightening up the different elements of the piece. It can also refer to the creation of different visual effects, such as the creation of the ripply water surface. One official definition is: "anactorinstanceofinterpretation, rendition, ordepiction,asofadramaticpartofa composition".
The term has been around for many, many years. The 3D term is a reference to the 2D. A rendering (rendition) is a representation. "The figure is rendered in clay." It also refers to the process of making the representation, or enhancing the representation. "This area is too rough, I need to do a little more rendering in the shadows."
In 3D, when you click the button, the computer is taking your instructions and creating a tight representation, thus you are "rendering" the image.
5 hours?! are you kidding me?! this would take me at least one week dude, you don't have to convince yourself not to think you are garbage for taking this long, thats nearly a time that is used for a speed painting, this is a fully rendered painting!