The word "chronometer" comes from the Ancient Greek words khrónos ("time") and métron ("measure"). 

The earliest known use of the word "chronometer" was in 1714 by William Derham, a Church of England clergyman and natural philosopher. 

Chronometers were originally designed for maritime navigation. Ordinary clocks were not useful at sea because of temperature changes and the ship's motion. 

The English clock maker John Arnold first used the word "chronometer" in 1779 to describe his pocket chronometer. 

The English clock maker John Arnold first used the word "chronometer" in 1779 to describe his pocket chronometer. 

Other words that contain the Greek root chron- include: Chronograph, Synchronized, Chronicle, Chronological, Anachronism, Chronic, and Crony.

Just the facts:

Chronometer Definition & Meaning - Merriam-Webster