Chronometer (the word)
Chronometer is a generic term for a precision timekeeper. Originally it meant a marine chronometer (the ship's timekeeper used for celestial navigation), and the word was tightly regulated. Today, in Switzerland, "chronometer" is a protected designation: a watch can only be sold as a chronometer if its movement has passed an accredited certification, and the only large-scale Swiss accredited body is COSC. So in normal usage, "chronometer" and "COSC certified" are synonyms in the Swiss market. In Japan or independent watchmaking the word can carry different (and looser) meanings.
COSC: the volume tier
COSC (Contrôle Officiel Suisse des Chronomètres) was founded in 1973 to consolidate the older cantonal observatory trials into a single industrial body. COSC tests uncased mechanical movements over 15 days at three temperatures (8, 23, 38 °C) and five positions, scoring seven criteria; the most important is mean daily rate of -4 to +6 seconds per day. About 2 million movements pass through COSC every year, mostly destined for Rolex, Omega, Breitling, Tudor, and a long tail of smaller brands. See our COSC explainer for the full process.
METAS: the in-case standard
COSC tests movements; the casing-up process can shift the rate. Buyers wanted a standard that certifies the finished, cased watch. Enter METAS, the Swiss Federal Institute of Metrology (the same body that certifies Switzerland's national kilogram). In 2014, Omega and METAS published the Master Chronometer standard. A Master Chronometer watch must already be COSC certified, then pass 8 additional METAS-administered tests on the cased watch: 0 to +5 sec/day after exposure to 15,000-gauss magnetic fields, on six positions, two power-reserve states, and across normal wearing temperatures. Master Chronometer = COSC + METAS, not METAS instead of COSC.
Tudor METAS: the same chassis
Since 2021, Tudor (Rolex's sister brand) sends some watches to METAS as well, branded as "Master Chronometer" with the same -0/+5 sec/day in case and 15,000-gauss spec. The Pelagos 39 and certain Black Bay variants are Master Chronometer; not every Tudor is. Tudor is unrelated to Omega here; both brands independently submit to the same METAS programme.
Other certifications worth knowing
Rolex Superlative Chronometer: Rolex's in-house cased-watch standard, -2 to +2 sec/day. Not METAS; Rolex tests every watch internally. Patek Philippe Seal: Patek's 2009 standard, -3 to +2 sec/day on cased watches >20mm, plus build-quality and finishing rules. Replaced Patek's use of the Geneva Seal. Geneva Seal (Poinçon de Genève): a finishing standard awarded to watches assembled in the canton of Geneva, with both finishing and timekeeping requirements (-1/+0 mean rate over 7 days). See our Geneva Seal explainer. Qualité Fleurier: a much rarer Swiss certification combining COSC, in-case 24h Chronofiable test, finishing review, and a 100% Swiss-made requirement.
What it means for your wallet
Cheapest: a non-certified mechanical (sub-CHF 1,000), regulated to "chronometer-grade" by the brand's eye but not individually tested. Mid-tier: COSC certified, individually tested, probably runs within -4/+6 sec/day for the first few years. Top mainstream: Master Chronometer (Omega/Tudor) or Superlative Chronometer (Rolex), where the cased watch is certified, not just the movement. The premium for Master Chronometer over plain COSC is typically negligible on a Seamaster or Speedmaster; you are essentially paying the same as a comparable Rolex Submariner. For complete grand-complication watches, certification is sometimes not claimed because the brand reputation alone is sufficient to command the price.