[b]Vernier Calipers [/b]
A caliper (British spelling also calliper) is a device used to measure the distance between two symmetrically opposing sides. 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, the caliper is then removed and the distance read by measuring between the tips with a measuring tool, such as a ruler.
Nomenclature variants (colloquial only)
A plurale tantum sense of the word "calipers" coexists in natural usage with the regular noun sense of "caliper". That is, sometimes a caliper is treated cognitively like a pair of glasses or a pair of scissors, resulting in a phrase such as "hand me those calipers" or "those calipers are mine" in reference to one unit. However, this usage is mostly colloquial and the regular noun sense of caliper usually dominates, especially in writing. There is no rigorous grammatical logic to defend the plurale tantum sense; it is just the way language sometimes naturally flows colloquially.
Also existing colloquially but not in formal usage is referring to a vernier caliper as a "vernier".