(noun) - In the context of drawing or painting, perspective is the technique an artist uses to create the illusion of three dimensions on a flat surface. Simply put, perspective is a means of fooling the viewer's eye. There only appears to be depth or receding space in that work.
An artist can manipulate shapes and sizes within a work, so that things that would look closer in 3-D look realistic in two dimensions. Now, however, we're veering off the topic of art history. If you'd like to learn more about how perspective can be used, please take a look at Introduction to Perspective Drawing.
"The sculptor, and the painter also, should be trained in these liberal arts: grammar, geometry, philosophy, medicine, astronomy, perspective, history, anatomy, theory of design, arithmetic." -- Lorenzo Ghiberti