Cyclops refers to an inset magnifying lens found on the crystal of some watches, notably Rolex.
Rolex pioneered the date window in watches and introduced an inset magnification lens to the crystal of the Rolex Datejust in 1953. This feature made the date easier to read and has spread throughout the company's model lineup in the decades since. Controversially, Rolex introduced the “cyclops” on the Rolex Submariner and Rolex Sea-Dweller in recent years, a move not all customers appreciate.
Many other watches include a “cyclops” magnifier as well.