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

Question

Most 35mm film camera lenses have a s***w 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".)

Answer 1

Darn fotoace beat me to the answer! Yes it's filter size.

Answer 2

The filter size.

And no, i didn't copy from the other two, i knew the answer.

Answer 3

Filter size ... that was easy