A kind of scale appearing in many modern bony fishes (order Osteichthys), typically ones with soft ray fins (i.e., without spines) such as trout. Cycloid scales tend to be smooth and either round or oval. They can be used to age a fish, since each year a growth ring (circulus) is formed around the scale as it grows.
Cycloid scales appeared after ganoid scales and before ctenoid scales.