A hygroscopic or hydroscopic substance is a substance that absorbs water readily from its surroundings.
An example is biodiesel, which absorbs water to about 1200 parts per million (PPM). Other hygroscopic substances include ethyl alcohol, methanol, glycerin, concentrated sulfuric acid and concentrated sodium hydroxide.
Those using hygroscopic materials should be advised to keep them in sealed containers, as they will absorb water from the air.
History
Nitrocellulose, unlike gunpowder, is not hydroscopic. Smokeless powder facilitated the development of semi-automatic and fully automatic firearms. This is because the hydroscopic residue of gunpowder would jam the mechanisms of such firearms.