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In the broadest sense, a cone is a shape with a circular base that tapers to a point. An everyday example is an ice cream cone. In the field of medicine, cone is sometimes used to refer to cone-shaped devices (usually made of metal) attached to equipment that use x-rays. The cone is used on the equipment to confine and/or focus a beam of x-rays. This definition of cone is also known as conus. Coni refers to more than one conus.

In the body, cone refers to a light-sensitive, outward directed, cone-shaped cell that is essential for color vision and sharp vision. Technically, the top part of the cone cell is called the cone. However, cone cells are often called cones. These cells are located in the retina. The retina is an area at the back of the eye that is sensitive to light. Cones are the only light-sensitive receptors in the fovea centralis, which is the area at the center of the retina specialized in sharp vision. There are three different types of cone cells in the retina based on the colors perceived: red cones, green cones, and blue cones. Other colors are perceived by stimulation of more than one type of cone.
Cones & Cone Cells of the Retina
Cones are contrasted with rods, which are other types of light-sensitive cells (shaped like rods) that help in detecting motion and information in low light but they are not sensitive to color. There are about 120 million rods in the eyes compared to 6 to 7 million cones. There are increasing numbers of rods mixed in with cones away from the center of the retina. Cone comes from the Greek word "konos" meaning "cone."