The concurrent is the second half of a synesthetic pairing, the experience the inducer produces. When a chord triggers a wash of violet, the violet is the concurrent, and because it is visual it also earns the name photism.
Concurrents are typically simpler than full scenes: colors, textures, shapes, movements, tastes. They are perceived rather than imagined, arrive without effort, and stay loyal to their inducers for decades, which is what consistency testing measures.
The inducer-concurrent vocabulary keeps research tidy across wildly different forms, from colored Mondays to bacon-flavored names.