Chronometry refers to the science and art of measuring time accurately, particularly in the design and function of precision timepieces.
Chronometry is the science of accurate time measurement and the study of the instruments used to measure it. In watchmaking, chronometry specifically refers to precision timekeeping and the verification of accuracy to certified standards. A watch described as having chronometric precision has been tested and certified to meet strict accuracy criteria—most commonly by COSC, which certifies mechanical movements achieving ±4-6 seconds per day as chronometers.
Chronometry is the science and measurement of time accuracy—how precisely a timepiece keeps time. Chronography (or chronograph) refers to the function of recording elapsed time, i.e., a stopwatch function. A chronometer is a precision watch certified for accuracy; a chronograph is a watch with a stopwatch. Despite similar-sounding names, they're entirely different concepts. A watch can be both a chronograph (has stopwatch) and a chronometer (certified accurate), or either independently.
COSC (Controle Officiel Suisse des Chronometres) is the Swiss standard, certifying movements achieving ±4-6 seconds per day across 15 days of testing. Rolex's in-house testing (Superlative Chronometer) targets ±2 seconds per day. Omega's METAS certification tests entire cased watches (not just movements) to 0/+5 seconds per day. Some manufacturers like Greubel Forsey and independent makers claim even tighter tolerances through proprietary standards.