On 35mm camera lenses, what is the name of the measurement for the thread diameter at the front of the lens?

Most 35mm film camera lenses have a screw thread on the inside of the lens housing at the front, which is used to attach filters or a sun hood. The diameter of this thread is usually marked on the front of the lens in millimeters, such as 49, 52, or 55, followed by an "o" with a diagonal slash through it. I need to know the name of this measurement. (It's not "focal length".)