A single piece of glass or plastic inside a camera lens. Most photographic lenses are built from several elements placed one behind the other along the same axis. Each element bends light in a certain way, and together they correct aberrations. Some elements are cemented together and work as one unit, called a group. Lens specifications often say, for example, "14 elements in 10 groups".
Do you have a better definition? Share it in the comments.
