Final answer:
The property of mercury used in a thermometer is its thermal expansion, which allows it to precisely measure temperature by expanding when heated.
Step-by-step explanation:
The property of a liquid such as mercury that is used in a liquid-in-glass thermometer is thermal expansion. When mercury is heated, it expands significantly more than the glass that encases it. This expansion of mercury is reliably proportional to temperature changes, which allows for precise temperature measurements as the mercury moves up and down the calibrated scale of the thermometer.
Unlike compressibility, which is not a significant factor due to the incompressible nature of liquids, thermal expansion provides the necessary sensitivity for temperature measurement in such thermometers.