The definition of an aneroid-barometer is an instrument with a needle that moves when a change occurs in atmospheric pressure.
An example of aneroid barometer is what a meteorologist would use to find out if rain is coming.
aneroid barometer definition by Webster's New World
a barometer with a needle connected by mechanical linkages and springs to a thin, hollow metal disk in which a partial vacuum is maintained: a change in atmospheric pressure changes the shape of the disk, thus moving the needle
A barometer consisting of a thin elastic disk covering a chamber that contains a partial vacuum. High atmospheric pressure pushes against the disk and causes it to bulge inward, while low pressure does not push as hard, allowing the disk to bulge outward. An aneroid barometer is smaller and more portable than a mercury barometer and, when used with a barograph, can record up to a week's worth of data. Aneroid barometers are used extensively in aviation as part of altimeters.