Readily absorbing moisture from the air, used to describe paper. A paper's primary constituents, cellulose fibers, have a strong affinity for water (which is how plants and trees—composed primarily of cellulose—survive), and changes on the relative humidity of a paper's environment can result in a gain or loss of moisture on the part of the paper, which affects its dimensional stability, or its ability to retain its original width and/or length. Such changes in size have deleterious effects on many printing processes. As paper gains moisture, the cellulose fibers increase primarily in width (but not as much in length), so paper tends to expand in the cross-grain direction (see Grain). (See also Dimensional Stability.)
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