What does "capillary" mean here?
"The typical laboratory thermometer contains a bulb (reservoir) of temperature-sensing liquid at the bottom; it is this portion of the thermometer that actually senses the temperature. The glass barrel of the thermometer above the liquid bulb contains a fine capillary opening in its center, into which the liquid rises as it expands in volume when heated. The capillary tube in the barrel of the thermometer has been manufactured to very strict tolerances, and it is very regular in cross-section along its length. This ensures that the rise in the level of liquid in the capillary tube as the thermometer is heated will be directly related to the temperature of the thermometer's surroundings. "

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Google dictionary says, "a tube that has an internal diameter of hairlike thinness."

So basically, it is a very small tube that branches off of the bottom of the thermometer.

Answer

As said capillary is a small tube. The smallest blood vessels in the body are also known as capillaries

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