A photism is the visual event itself: the scarlet that arrives with the trumpet, the drifting form that accompanies a voice. In the inducer-concurrent vocabulary, photisms are visual concurrents.
The term entered science in the 19th century through studies of sound-triggered light sensations, and it has named the phenomenon ever since. Photisms are consistent per person, specific to fine details of the trigger, and involuntary.
We wrote the full story, including where the word comes from and why our software carries it, in What is a photism?