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the water content of physical bodies. Moisture depends upon the relative humidity of the surrounding environment, upon the nature of the substance, and, in solids, also upon the degree of porosity and the size of the particles—that is, the general dimensions of the interior and exterior surfaces of the objects. The concept of moisture does not include the content of crystalline water or of chemically bound, so-called constitutional, water—for example, the H2O that is produced only during chemical decomposition.
Moisture is usually characterized by the quantity of water in a substance, expressed in the percentage of the original mass of the moist substance (moisture by mass) or its volume (volume moisture). Moisture can also be characterized as the moisture content, or absolute moisture—the quantity of water relative to a unit of weight of the dry portion of the substance. It is of great significance to the national economy to establish the acceptable degree of moisture for many products and materials; such substances as grain or cement are only useful up to a certain moisture content. The vitality of animal and plant organisms is possible only within certain limits of moisture and relative humidity of the surrounding atmosphere.