What light is bent through to make a thing visible. Also a chosen way of looking — every lens shows by excluding.
Latin lens (a lentil, the flattened seed); 17th-century opticians named the ground-glass disc for its shape, and the word carries the lentil's form into every meaning of a chosen way of looking since.
Every lens makes something visible by choosing what it leaves out — clarity purchased with a deliberate narrowing.