A fluorogen is a ligand (fluorogenic ligand) which is not itself fluorescent, but when it is bound by a specific protein or RNA structure becomes fluorescent.
For instance, FAST is a variant of photoactive yellow protein which was engineered to bind chemical mimics of the GFP tripeptide chromophore.
Likewise, the spinach aptamer is an engineered RNA sequence which can bind GFP chromophore chemical mimics, thereby conferring conditional and reversible fluorescence on RNA molecules containing the sequence.