A tachymeter is a scale, usually printed on the bezel, that turns elapsed seconds into a rate per hour. Start the timing hand as something passes a mark, and stop it at the next mark. The number under the hand is the rate. The scale is a reading aid. It does not grade the watch.
The word is defined here so a reference page can stay on one model. The glossary index remains at Glossary. A chronograph, the function that makes the scale useful, is described under Complications. A bezel that someone later changed is a different note, under Modifications.
A library page is written by Watch Notes. A photo post is one person’s notes. Neither authenticates a watch or speaks for the name on the dial.