The Collaborative International Dictionary of English v.0.48:

Hygroscopic \Hy`gro*scop"ic\, a. [Cf. F. hygroscopique.]
   1. Of or pertaining to, or indicated by, the hygroscope; not
      readily manifest to the senses, but capable of detection
      by the hygroscope; as, glass is often covered with a film
      of hygroscopic moisture.
      [1913 Webster]

   2. Having the property of readily inbibing moisture from the
      atmosphere, or of the becoming coated with a thin film of
      moisture, as glass, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hygroscopic
    adj 1: absorbing moisture (as from the air)