Lens that is convex on one side and concave on the other (8)
I believe the answer is:
meniscus
(Other definitions for meniscus that I've seen before include "Curve on fluid surface in a tube" , "Curve on liquid surface" , "Cartilage between bones that meet at a joint" , "Curved upper surface of liquid in a tube" , "Overfilled glass shows it!" .)