1. (n.) A crescent.

2. (n.) A lens convex on one side and concave on the other.

3. (n.) An interarticular synovial cartilage or membrane; esp., one of the intervertebral synovial disks in some parts of the vertebral column of birds.

achromatic lens astigmatic lens burning glass camera coated lens concave lens concavo-convex lens condenser convex lens crescent crescent moon eyeglass eyepiece glass half-moon hand lens horseshoe lens lunula lunule magnifier magnifying glass object glass objective objective prism ocular prism reader reading glass scythe semicircle sickle telephoto lens toric lens varifocal lens zoom lens