The term "rendering" is actually an old one, it just became more prevalent when 3D came out. A rendering is typically a picture or painting created to represent something, like a building. A 3D Rendering is simply an image that happens to have been created by a 3D Artist on a computer.
Each rendering can be generated to a difference size, where the term "high-resolution" comes into play. The 3D artist sets the size for the rendering and the computer processes it out, which could take minutes or hours. The larger the rendering, the longer the time needed to process.