An optical artifact caused by sufficiently intense light entering a lens. This light reflects and scatters in a way that leads to it "leaking" into the image, appearing as bright patches of various shapes and sizes in the image.
Lens flare can appear in three different ways:
Bright streaks radiating from the light source.
Circles or polygons of various pale colors and sizes centered on a line pointing to the light source. These shapes are multiple images of the camera's diaphragm aperture, also known as an iris. Cameras with 5-bladed irises will produce pentagons, 6-bladed hexagons, etc., with more blades producing a progressively more circular shape. These can appear even if the light source is outside the field of view.
Anamorphic lenses are a special type of lens that have been used extensively in movie cameras since the 1950's. They produce a characteristic lens flare similar to the first, except streaks are strictly horizontal. Streaks in other directions are either not present or much fainter. Because of its use in the film industry, for most people this type of lens flare has a very strong association with movies, and this effect is often simulated to impart a cinematic quality.
Rainbow-colored arcs that partially or entirely encircle the light source. These are called halos, and are not camera artifacts. Like rainbows, they are caused by light being reflected and refracted by water droplets or ice crystals in the atmosphere.
bokeh. Bokeh is affected by the shape and size of a camera's iris in the same way as the second type of lens flare. However bokeh does not appear in a line, instead each spot corresponds to a single, out-of-focus light source.
"Love bubbles": A visual trope in anime-style images, consisting of translucent bubbles appearing throughout a picture. These give an illustration a "lovey-dovey" or "dreamy" feeling. While sometimes appearing similar to lens flare, they are not associated with any light source and can appear even in very dark pictures.
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