The ability of a substance to absorb moisture from air. For example, sodium hydroxide pellets are so hygroscopic that they dissolve in the water they absorb from the air.
• (n.) The property possessed by vegetable tissues of absorbing or discharging moisture according to circumstances.
(from the article `wood`) Wood can absorb water as a liquid, if in contact with it, or as vapour from the surrounding atmosphere. Although wood can absorb other liquids and ...
Hy`gro·sco·pic'i·ty noun (Botany) The property possessed by vegetable tissues of absorbing or discharging moisture according to circumstances. 
Hygroscopicity is the taking up and giving out of moisture to and from the atmosphere.
hygroscopicity 1. The quality, or degree, of absorbing moisture. 2. The property possessed by vegetable tissues of absorbing or discharging moisture according to circumstances.