Select the most appropriate one-word substitution for the given group of words. 

A highly accurate timepiece 

Chronometer

The most appropriate one-word substitution for "a highly accurate timepiece" is chronometer.

• Chronometer is a highly accurate timepiece that is used to measure time.
• Barometer is an instrument that measures atmospheric pressure.
• Lactometer is an instrument that measures the density of milk.
• Thermometer is an instrument that measures temperature.

The word chronometer comes from the Greek words "chronos," meaning "time," and "metron," meaning "measure." A chronometer is typically used to measure time at sea, where the effects of gravity and temperature can affect the accuracy of other timepieces. Chronometers are very accurate and are often used in navigation and scientific research.