Rub closed eyes and the blooms of light that follow are phosphenes: the visual system reporting activity that never came through the pupil. Pressure, magnetic stimulation, and sometimes nothing at all can produce them.
Phosphenes and photisms are cousins worth keeping apart. A phosphene needs no external trigger and carries no information; a photism is caused by a stimulus in another sense and tracks it precisely. Both demonstrate the same underlying fact, though: visual experience is generated by the brain, and light is only its usual, not its only, cause.
The word is also 19th-century coinage, from the Greek for light and to show.