What a chronometer means
A chronometer is a high-precision timekeeper certified by an independent authority or described under the applicable rules. In Switzerland, COSC tests qualifying movements and watches against defined criteria. Its mechanical wristwatch certification is based on ISO 3159 and involves testing across positions and temperatures over a prescribed period.
Certification is a snapshot against a test standard, not a promise that a watch will never need regulation or that every example will have identical performance in every environment. Look at the named certifier and the actual standard, not just the word printed on a dial.
What a chronograph does
A chronograph lets the wearer start, stop and reset an elapsed-time display, commonly using pushers on the case. Depending on the design, elapsed seconds, minutes or hours appear on a central hand or subdials. A tachymeter scale is often paired with a chronograph but is not required for the function.
Chronograph movements may be mechanical or quartz. Operating procedures differ, so consult the manual, particularly before using pushers underwater or operating a chronograph with a screw-down control.
One watch can be both
The terms describe separate things: certification status and a timing complication. A chronograph can be chronometer-certified if it passes a relevant certification, and a simple time-only watch can be a chronometer without having a chronograph.
When comparing watches, ask what is certified (movement or finished watch), who performed the test, which standard applies and what the chronograph measures. This makes marketing terms easier to interpret accurately.