instrument used to measure the diameter of the trunk of a tree.

Merriam-Webster Online Dictionary
caliper (noun)
a) any of various measuring instruments having two usually adjustable arms, legs, or jaws used especially to measure diameter or thickness - usually used in plural a pair of calipers
b) a device for pressing a frictional material (as a brake pad) against the sides of a rotating wheel or disc
thickness especially of paper, cardboard, or a tree
caliper (verb)
transitive verb
to measure by or as if by - calipers
caliper (Wikipedia)
Caliper detail view

A caliper (British spelling also calliper, or in plurale tantum sense a pair of calipers) is a device used to measure the distance between two opposite sides of an object. Many types of calipers permit reading out a measurement on a ruled scale, a dial, or a digital display. But a caliper can be as simple as a compass with inward or outward-facing points. The tips of the caliper are adjusted to fit across the points to be measured and then the caliper is then removed and the distance read by measuring between the tips with a measuring tool, such as a ruler.

It is used in many fields such as mechanical engineering, metalworking, forestry, woodworking, science and medicine.

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