lens define
Medical Definition of LENS. 1. : a curved piece of glass or plastic used singly or combined in eyeglasses or an optical instrument (as a microscope) for forming an image by focusing rays of light.2.: a device for directing or focusing radiation other than light (as sound waves, radio microwaves, or electrons).3.. : a curved piece of glass or plastic used singly or combined in eyeglasses or an optical instrument (as a microscope) for forming an image by focusing rays of light. 2.