In the context of contact lenses, BC stands for Base Curve, which is the measurement of the back curvature of the contact lens. It is an important part of a contact lens prescription, and it must be included along with other criteria such as lens power and diameter. The base curve is used to determine the type of fit required for the lens to meet the curve of the eye, and it is usually written in millimeters or sometimes with the words flat, median, or steep. The BC value typically ranges from about 8.0 to 10.0, and the doctor fits the lens with the curvature most appropriate for the patients eye. If a brand only comes in one base curve, the prescription will not contain a BC value.