Lens Flare
Stray light scattered inside a lens by a bright source, appearing as streaks, polygons or a general wash across the image.
In depth
Flare has two forms: discrete ghosts, which are reflections between element surfaces, and veiling glare, which is diffuse scatter that lifts the blacks. Ghost shapes echo the aperture blades, which is why a stopped-down lens produces hexagonal or heptagonal artefacts. It can be a fault or a deliberate device, and cinematographers frequently choose lenses for how they flare.
Examples in practice
A summer sun just outside the frame throws a chain of coloured polygons diagonally across a portrait.