Perspective is a physical term that refers to the way the eye perceives objects based on their spatial relation. In terms of perspective, objects closer to the eye appear larger than those that are farther away.

For example, a quarter in your hand can appear bigger than a plane high up in the sky (even though, logically speaking, the airplane is clearly much larger than the quarter). Similarly, two parallel lines, when they go off into the distance, will appear to the viewer to touch far off, even though the viewer knows they are parallel.
By using different lens to a camera, the photographer can alter the natural perspective of a given scene. Fisheye lenses, close-up lenses and distortion lenses are all tools that play with perspective. While photographers may play with perspective in their photos for aesthetic effect, perspective is more often associated with the viewer’s experience looking at the finished photograph.