occluder
Technical term in ophthalmology or optometry for a device that blocks vision in one eye, used during eye examinations or vision therapy.
During an eye exam, one eye is covered with an occluder.
Compound of 遮 (sha, 'to block') + 眼 (gan, 'eye') + 子 (shi, a suffix for small objects or instruments). The exact origin is uncertain, but it is a modern technical coinage.