What is really a Chronometer?

There is a lot of confusion about this term in horology.
Many people think a Chronometer is simply a watch used to precisely measure short time intervals, typically during sports events.
Indeed, a watch is classified as a "Chronometer" if it satisfies different conditions. Let's start talking about the C.O.S.C. (Contrôle Officiel Suisse des Chronomètres). It's a Swiss institute created to examine watches to be called later "Chronometers". The C.O.S.C. only is enabled to give this official qualification.
It will be granted if the performance is particularly good: for example, the mean daily rate in different positions must be between - 3 and + 6 sec/day in different positions and temperatures (8, 23, 39° C) over a period of 15 days for a large size wristwatch.
The Chronometer certificate is quite expensive: the C.O.S.C. doesn't work for free! There are two different kinds: with or without details of trial results. The former is more expensive and rarely seen on today's watches, whose price is heavily influenced by certification.
You can make an idea of it looking at Rolex pricelist. Rolex sells similar watches with or without certificate.
It must be remembered that the certificate qualifies undoubtedly a movement as "excellent": this greatly increases the prestige of a particular watch. In high-end horology this fact is very important.
Here you can see the screenshots of two images taken from a certificate gained by a Rolex wristwatch.
Click on them to view the full size picture (large files, loading may require some time):