The story
So called by its inventor, John Smeaton, in the mid-18th century, because it consisted in part of a pear-shaped glass vessel.
Etymology from Wiktionary, CC BY-SA 4.0.
pear gauge
Today it means: A kind of gauge for measuring the exhaustion (creation of a vacuum) of an air pump receiver (airtight vessel from which air is pumped in order to form a vacuum). Full definition →