A flashing sign is a sign that contains a sequential flashing light source where the period of time of illumination is equal to the period of non-illumination, and is used solely to attract attention in a non-informative way.
The light can be intermittent or flashing, scintillating, blinking or traveling which give an illusion of flashing or intermittence or as an animation.
References
See also Neon sign Flicker vertigo Photosensitivity