Lens Flare
Lens flare is caused by light hitting the camera's lens and reflecting around off the internal elements of the lens. Generally lens flare is unwanted, although it can be used to good creative effect in some instances. You will often find lens flare in your photos when shooting with a bright light source (such as the sun) in or just outside the frame.
My Assistant by Jonathan Kos-Read on flickr (licensed CC-BY-ND)
